This patch makes MICROPY_PY_BUILTINS_SET compile-time option fully
disable the builtin set object (when set to 0).  This includes removing
set constructor/comprehension from the grammar, the compiler and the
emitters.  Now, enabling set costs 8168 bytes on unix x64, and 3576
bytes on stmhal.

Viper can now do: ptr8(buf)[0], which loads a byte from a buffer using
machine instructions.

This should pretty much resolve issue #50.

Viper can now do the following:

def store(p:ptr8, c:int):
    p[0] = c

This does a store of c to the memory pointed to by p using a machine
instructions inline in the code.

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: ...

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

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.

Towards resolving issue #50.

Native x86-64 now has 3 locals in registers.

Fix some bugs with x86 stack and saving registers correctly.

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

Addresses issue #838.

Viper functions can now be annotated with the type of their arguments
and return value.  Eg:

@micropython.viper
def f(x:int) -> int:
    return x + 1

See discussion in issue #50.

Native emitter can now compile try/except blocks using nlr_push/nlr_pop.
It probably only works for 1 level of exception handling.  It doesn't
work on Thumb (only x64).

Native emitter can also handle some additional op codes.

With this patch, 198 tests now pass using "-X emit=native" option to
micropython.

It defines types used by all other headers.

Fixes #691.

Needed to pop the iterator object when breaking out of a for loop.  Need
also to be careful to unwind exception handler before popping iterator.

Addresses issue #635.

Addresses issue #627.

This patch simplifies the glue between native emitter and runtime,
and handles viper code like inline assember: return values are
converted to Python objects.

Fixes issue #531.