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.

Also, implement for unix port.

While extension to file.readinto() definition of CPython, the additional arg
is similar to what in CPython available in socket.recv_into().

Also, usocket.readinto(). Known issue is that .readinto() should be available
only for binary files, but micropython uses single method table for both
binary and text files.

Just like they handled in other read*(). Note that behavior of readline()
in case there's no data when it's called is underspecified in Python lib
spec, implemented to behave as read() - return None.

This is CPython-compatible convention established yet in acb13886.

With this fix, file_long_read now passes.

With unicode enabled, this patch allows reading a fixed number of
characters from text-mode streams; eg file.read(5) will read 5 unicode
chars, which can made of more than 5 bytes.

For an ASCII stream (ie no chars > 127) it only needs to do 1 read.  If
there are lots of non-ASCII chars in a stream, then it needs multiple
reads of the underlying object.

Adds a new test for this case.  Enables unicode support by default on
unix and stmhal ports.

See discussion in issue #50.

- rearrange/add definitions that were not there so it's easier to compare both
- use MICROPY_PY_SYS_PLATFORM in main.c since it's available anyway
- define EWOULDBLOCK, it is missing from ingw32

Getting a length of string may be expensive, depending on the underlying
implementation.