Native x86-64 now has 3 locals in registers.

Fix some bugs with x86 stack and saving registers correctly.

This is required to deal with, well, packed C structs containing pointers.

Type representing signed size doesn't have to be int, so use special value
which defaults to SSIZE_MAX, but as it's not defined by C standard (but rather
by POSIX), allow ports to set it.

For the sake of older versions of gcc (and other compilers), don't use
the #warning CPP directive, nor the -Wno-error=cpp option.

Also, fix a strict alias warning in modffi.c for older compilers, and
add a test for ffi module.

Addresses issue #847.

Previously, mpz was restricted to using at most 15 bits in each digit,
where a digit was a uint16_t.

With this patch, mpz can use all 16 bits in the uint16_t (improvement
to mpn_div was required).  This gives small inprovements in speed and
RAM usage.  It also yields savings in ROM code size because all of the
digit masking operations become no-ops.

Also, mpz can now use a uint32_t as the digit type, and hence use 32
bits per digit.  This will give decent improvements in mpz speed on
64-bit machines.

Test for big integer division added.

Script is equivalent, but now also runs under ancient Python 2.6.
Goes part way to addressing issue #847.

Code-info size, block name, source name, n_state and n_exc_stack now use
variable length encoded uints.  This saves 7-9 bytes per bytecode
function for most functions.

And move the MAP_ANON redefinition from py/asmx64.c to unix/alloc.c.

If not set, MICROPY_EMIT_X64 is set only if on x86-64 machine.

If not set, MICROPY_GCREGS_SETJMP is set when on MIPS.

Add allocation macros (per platform) and ARM cache flush

Fixes issue #840

Declare do_str() function before the implementation

This ensures that GCC does not discard the do_str implementation in
some cases eg when compiling tests with debug enabled:
  make RUN_TESTS=1 DEBUG=1

Allows to create socket objects that support TCP and UDP in server and
client mode.  Interface is very close to standard Python socket class,
except bind and accept do not work the same (due to hardware not
supporting them in the usual way).

Not compiled by default.  To compile this module, use:
make MICROPY_PY_WIZNET5K=1

Update teensy README.md file

Thanks to Artur Wroblewski for some suggested changes.
I also added the TIPs section at the end while I was updating.

Part of code cleanup, working towards resolving issue #50.