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.

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.

Fixes issue #840

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

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

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

Part of code cleanup, to resolve issue #50.

Part of code cleanup, towards resolving issue #50.

Addressing issue #50, still some way to go yet.

Found this bug by running unix/ tests with DEBUG=1 enabled when
compiling.

Addresses issue #838.