This patch makes configurable, via MICROPY_QSTR_BYTES_IN_HASH, the
number of bytes used for a qstr hash.  It was originally fixed at 2
bytes, and now defaults to 2 bytes.  Setting it to 1 byte will save
ROM and RAM at a small expense of hash collisions.

TODO: Contributed code in modesp incorrectly uses private gc_* API.

Simplelink's socket send checks for the size of the packet and sends
it in chunks if the size is too large.

Previous to this patch all interned strings lived in their own malloc'd
chunk.  On average this wastes N/2 bytes per interned string, where N is
the number-of-bytes for a quanta of the memory allocator (16 bytes on 32
bit archs).

With this patch interned strings are concatenated into the same malloc'd
chunk when possible.  Such chunks are enlarged inplace when possible,
and shrunk to fit when a new chunk is needed.

RAM savings with this patch are highly varied, but should always show an
improvement (unless only 3 or 4 strings are interned).  New version
typically uses about 70% of previous memory for the qstr data, and can
lead to savings of around 10% of total memory footprint of a running
script.

Costs about 120 bytes code size on Thumb2 archs (depends on how many
calls to gc_realloc are made).

inet_pton supports both ipv4 and ipv6 addresses. Interface is also extensible
for other address families, but underlying libc inet_pton() function isn't
really extensible (e.g., it doesn't return length of binary address, i.e. it's
really hardcoded to AF_INET and AF_INET6). But anyway, on Python side, we could
extend it to support other addresses.

Required to implement UDP servers.

sendto() turns out to be mandatory function to work with UDP. It may seem
that connect(addr) + send() would achieve the same effect, but what connect()
appears to do is to set source address filter on a socket to its argument.
Then everything falls apart: socket sends to a broad-/multi-cast address,
but reply is sent from real peer address, which doesn't match filter set
by connect(), so local socket never sees a reply.

Treating it as signed lead to buffer overflow for ports >= 32768.

This usually allows to get just a single address entry.

This requires root access. And on recent Linux kernels, with
CONFIG_STRICT_DEVMEM option enabled, only address ranges listed in
/proc/iomem can be accessed. The above compiled-time option can be
however overriden with boot-time option "iomem=relaxed".

This also removed separate read/write paths - there unlikely would
be a case when they're different.

Update pydfu.py to match with the version from openmv.

I just updated the openmv version to work with both of the
PyUSB 1.0.0.b1 and 1.0.0.b2

See: https://github.com/walac/pyusb/blob/master/ReleaseNotes.rst

I checked the entire codebase, and every place that vstr_init_len
was called, there was a call to mp_obj_new_str_from_vstr after it.

mp_obj_new_str_from_vstr always tries to reallocate a new buffer
1 byte larger than the original to store the terminating null
character.

In many cases, if we allocated the initial buffer to be 1 byte
longer, we can prevent this extra allocation, and just reuse
the originally allocated buffer.

Asking to read 256 bytes and only getting 100 will still cause
the extra allocation, but if you ask to read 256 and get 256
then the extra allocation will be optimized away.

Yes - the reallocation is optimized in the heap to try and reuse
the buffer if it can, but it takes quite a few cycles to figure
this out.

Note by Damien: vstr_init_len should now be considered as a
string-init convenience function and used only when creating
null-terminated objects.

Previous to this patch, if "abcd" and "ab" were possible completions
to tab-completing "a", then tab would expand to "abcd" straight away
if this identifier appeared first in the dict.

Drops Thumb2 arch size by 24 bytes.