This included a bit of restructuring of the assembler backends.  Note
that the ARM backend is missing a few functions and won't compile.

Allows things like: if 1: ...

It seems that newer versions of clang don't like the .bss directive, so
we don't use it for OSX.

Addressing issues #865 and #875.

The JSON specs are relatively flexible and allow us to use one function
to print strings, be they ascii, bytes or utf-8 encoded.

Addresses issue #864.

It seems most sensible to use size_t for measuring "number of bytes" in
malloc and vstr functions (since that's what size_t is for).  We don't
use mp_uint_t because malloc and vstr are not Micro Python specific.

mp_parse_node_free now frees the memory associated with non-interned
strings.  And the parser calls mp_parse_node_free when discarding a
non-used node (such as a doc string).

Also, the compiler now frees the parse tree explicitly just before it
exits (as opposed to relying on the caller to do this).

Addresses issue #708 as best we can.

Stack is full descending and must be 8-byte aligned.  It must start off
pointing to just above the last byte of RAM.

Previously, stack started pointed to last byte of RAM (eg 0x2001ffff)
and so was not 8-byte aligned.  This caused a bug in combination with
alloca.

This patch also updates some debug printing code.

Addresses issue #872 (among many other undiscovered issues).

Also add start of ujson module with dumps implemented.  Enabled in unix
and stmhal ports.  Test passes on both.

Heap RAM was being allocated to print dicts and do some other types of
iterating.  Now these iterations use 1 word of state on the stack.

Deleting elements from a dict was not allowing the value to be reclaimed
by the GC.  This is now fixed.

This gets ARM native emitter working againg and addresses issue #858.

sys.exit always raises SystemExit so doesn't need a special
implementation for each port.  If C exit() is really needed, use the
standard os._exit function.

Also initialise mp_sys_path and mp_sys_argv in teensy port.

Tested and working on unix and pyboard.

Eventually, viper wants to be able to use raw pointers to strings and
arrays for efficient access.  But for now, let's just load strings as a
Python object so they can be used as normal.  This will anyway be
compatible with eventual intended viper behaviour.

Addresses issue #857.

Also use less stack space when printing single precision float.

Addition of asinf and acosf addresses issue #851.

Addresses issue #848.

Towards resolving issue #50.

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.