The vstr argument to the calls to vstr_add_len are dynamically allocated
(ie fixed_buf=false) and so vstr_add_len will never return NULL.  So
there's no need to check for it.  Any out-of-memory errors are raised by
the call to m_renew in vstr_ensure_extra.

Per POSIX, this is EINVAL, so raises OSError(EINVAL).

There were several different spellings of MicroPython present in comments,
when there should be only one.

Its addition was due to an early exploration on how to add CPython-like
stream interface. It's clear that it's not needed and just takes up
bytes in all ports.

Saves the following number of bytes of code space: 176 for bare-arm, 352
for minimal, 272 for unix x86-64, 140 for stmhal, 120 for esp8266.

This is an often used code pattern, and its use reduces code size of the
core by about 100 bytes.

vstr_extend will now only return NULL if the vstr is a fixed buffer, which
in this case it is not.

In current state `mp_get_stream_raise` assumes that `self_in` is an object
and always performs a pointer derefence which may cause a segfault.

This function shall throw an exception whenever `self_in` does not implement
a stream protocol, that includes qstr's and numbers.

fixes #2331

To filter out even prototypes of mp_stream_posix_*() functions, which
require POSIX types like ssize_t & off_t, which may be not available in
some ports.

Previoussly such read() and write() methods were used by modussl_axtls,
move to py/stream for reuse.

3-arg form:

stream.write(data, offset, length)

2-arg form:

stream.write(data, length)

These allow efficient buffer writing without incurring extra memory
allocation for slicing or creating memoryview() object, what is
important for low-memory ports.

All arguments must be positional. It might be not so bad idea to standardize
on 3-arg form, but 2-arg case would need check and raising an exception
anyway then, so instead it was just made to work.

It's now used for more than just stream protocol (e.g. pin protocol), so
don't use false names.

Both read and write operations support variants where either a) a single
call is made to the undelying stream implementation and returned buffer
length may be less than requested, or b) calls are repeated until requested
amount of data is collected, shorter amount is returned only in case of
EOF or error.

These operations are available from the level of C support functions to be
used by other C modules to implementations of Python methods to be used in
user-facing objects.

The rationale of these changes is to allow to write concise and robust
code to work with *blocking* streams of types prone to short reads, like
serial interfaces and sockets. Particular object types may select "exact"
vs "once" types of methods depending on their needs. E.g., for sockets,
revc() and send() methods continue to be "once", while read() and write()
thus converted to "exactly" versions.

These changes don't affect non-blocking handling, e.g. trying "exact"
method on the non-blocking socket will return as much data as available
without blocking. No data available is continued to be signaled as None
return value to read() and write().

From the point of view of CPython compatibility, this model is a cross
between its io.RawIOBase and io.BufferedIOBase abstract classes. For
blocking streams, it works as io.BufferedIOBase model (guaranteeing
lack of short reads/writes), while for non-blocking - as io.RawIOBase,
returning None in case of lack of data (instead of raising expensive
exception, as required by io.BufferedIOBase). Such a cross-behavior
should be optimal for MicroPython needs.

Saves 16 bytes of code.

Also, use mp_obj_get_int_truncated to allow integers as big as a machine
word to be passed as the value.

Will call underlying C virtual methods of stream interface. This isn't
intended to be added to every stream object (it's not in CPython), but
is convenient way to expose extra operation on Python side without
adding bunch of Python-level methods.

Spools entire output buffer to a blocking stream (chunk by chunk if
needed).

With this patch the n_args parameter is changed type from mp_uint_t to
size_t.

This allows the mp_obj_t type to be configured to something other than a
pointer-sized primitive type.

This patch also includes additional changes to allow the code to compile
when sizeof(mp_uint_t) != sizeof(void*), such as using size_t instead of
mp_uint_t, and various casts.

mp_obj_get_int_truncated will raise a TypeError if the argument is not
an integral type.  Use mp_obj_int_get_truncated only when you know the
argument is a small or big int.

This cleans up vstr so that it's a pure "variable buffer", and the user
can decide whether they need to add a terminating null byte.  In most
places where vstr is used, the vstr did not need to be null terminated
and so this patch saves code size, a tiny bit of RAM, and makes vstr
usage more efficient.  When null termination is needed it must be
done explicitly using vstr_null_terminate.

After vstr refactor. Fixes #1084.

With this patch str/bytes construction is streamlined.  Always use a
vstr to build a str/bytes object.  If the size is known beforehand then
use vstr_init_len to allocate only required memory.  Otherwise use
vstr_init and the vstr will grow as needed.  Then use
mp_obj_new_str_from_vstr to create a str/bytes object using the vstr
memory.

Saves code ROM: 68 bytes on stmhal, 108 bytes on bare-arm, and 336 bytes
on unix x64.

This patch allows to reuse vstr memory when creating str/bytes object.
This improves memory usage.

Also saves code ROM: 128 bytes on stmhal, 92 bytes on bare-arm, and 88
bytes on unix x64.

Addresses issue #1022.

mp_obj_int_get_truncated is used as a "fast path" int accessor that
doesn't check for overflow and returns the int truncated to the machine
word size, ie mp_int_t.

Use mp_obj_int_get_truncated to fix struct.pack when packing maximum word
sized values.

Addresses issues #779 and #998.