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.

By measuring SD card addresses in blocks and not bytes, one can get away
with using 32-bit numbers.

This patch also uses proper atomic lock/unlock around SD card
read/write, adds SD.info() function, and gives error code for failed
read/writes.

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.

Add LIS3DSH accelometer support to staccel.py

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.