Addresses issue #1022.

This patch adds a configuration option (MICROPY_CAN_OVERRIDE_BUILTINS)
which, when enabled, allows to override all names within the builtins
module.  A builtins override dict is created the first time the user
assigns to a name in the builtins model, and then that dict is searched
first on subsequent lookups.  Note that this implementation doesn't
allow deleting of names.

This patch also does some refactoring of builtins code, creating the
modbuiltins.c file.

Addresses issue #959.

mp_lexer_t type is exposed, mp_token_t type is removed, and simple lexer
functions (like checking current token kind) are now inlined.

This saves 784 bytes ROM on 32-bit unix, 348 bytes on stmhal, and 460
bytes on bare-arm.  It also saves a tiny bit of RAM since mp_lexer_t
is a bit smaller.  Also will run a bit more efficiently.

Going from MICROPY_ERROR_REPORTING_NORMAL to
MICROPY_ERROR_REPORTING_TERSE now saves 2020 bytes ROM for ARM Thumb2,
and 2200 bytes ROM for 32-bit x86.

This is about a 2.5% code size reduction for bare-arm.

Undefined behavior in C, needs explicit check.

This allows to implement KeyboardInterrupt on unix, and a much safer
ctrl-C in stmhal port.  First ctrl-C is a soft one, with hope that VM
will notice it; second ctrl-C is a hard one that kills anything (for
both unix and stmhal).

One needs to check for a pending exception in the VM only for jump
opcodes.  Others can't produce an infinite loop (infinite recursion is
caught by stack check).

This should be pretty compliant with CPython, except perhaps for some
corner cases to do with globals/locals context.

Addresses issue #879.

This has benefits all round: code factoring for parse/compile/execute,
proper context save/restore for exec, allow to sepcify globals/locals
for eval, and reduced ROM usage by >100 bytes on stmhal and unix.

Also, the call to mp_parse_compile_execute is tail call optimised for
the import code, so it doesn't increase stack memory usage.

This should pretty much resolve issue #50.

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.

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).

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.

This way, the native glue code is only compiled if native code is
enabled (which makes complete sense; thanks to Paul Sokolovsky for
the idea).

Should fix issue #834.

qstr_init is always called exactly before mp_init, so makes sense to
just have mp_init call it.  Similarly with
mp_init_emergency_exception_buf.  Doing this makes the ports simpler and
less error prone (ie they can no longer forget to call these).

See discussion in issue #50.

As stack checking is enabled by default, ports which don't call
stack_ctrl_init() are broken now (report RuntimeError on startup). Save
them trouble and just init stack control framework in interpreter init.

One thing is wanting to do 1 / 2 and get something else but 0, and quite
another - doing rocket science ;-).

Benefits: won't crash baremetal targets, will provide Python source location
when not implemented feature used (it will no longer provide C source
location, but just grep for error message).

- Move the includes for alloca() intp mpconfigport.h

This saves 4 words of stack space per Python call.

This allows to have multiple "optimization" levels (CPython has two
(-OO removes docstrings), we can have more).

This renames:
MICROPY_PY_FROZENSET -> MICROPY_PY_BUILTINS_FROZENSET
MICROPY_PY_PROPERTY -> MICROPY_PY_BUILTINS_PROPERTY
MICROPY_PY_SLICE -> MICROPY_PY_BUILTINS_SLICE
MICROPY_ENABLE_FLOAT -> MICROPY_PY_BUILTINS_FLOAT

See issue #35 for discussion.

Also unifies use of SMALL_INT_FITS macro across parser and runtime.

Addresses issue #627.