Only available when MICROPY_CAN_OVERRIDE_BUILTINS is enabled.

Cortex-M0, M0+ and M1 only have ARMv6-M Thumb/Thumb2 instructions.  M3,
M4 and M7 have a superset of these, named ARMv7-M.  This patch adds a
config option to enable support of the superset of instructions.

Addresses issue #1182.

See issue #1500.

Addresses #1510.

The whole current port gets slurped into a static lib named
"libmicropython.a". Maybe that's not ideal, but at least something
to start with.

It makes much more sense to do constant folding in the parser while the
parse tree is being built.  This eliminates the need to create parse
nodes that will just be folded away.  The code is slightly simpler and a
bit smaller as well.

Constant folding now has a configuration option,
MICROPY_COMP_CONST_FOLDING, which is enabled by default.

Using 'P' format specifier (matches struct module). This is another shortcut
for FFI, just as previously introduced "array of objects" ('O').

The usual cause would be that a cross-compiler for a port is not in PATH.

This patch eliminates the need for a nested parse node for assignments
and keyword arguments.  It saves a little bit of RAM when parsing.

Also adds tests specifically for testing constant folding.

With this patch parse nodes are allocated sequentially in chunks.  This
reduces fragmentation of the heap and prevents waste at the end of
individually allocated parse nodes.

Saves roughly 20% of RAM during parse stage.

Python semantics are that rhs of shift must be non-negative, so there's
no need to handle negative values in the underlying mpz implementation.

This patch adds more fine grained error message control for errors when
parsing integers (now has terse, normal and detailed).  When detailed is
enabled, the error now escapes bytes when printing them so they can be
more easily seen.

It's relatively small (between 44 and 56 bytes) and helps to reduce heap
pressure and fragmentation during compilation.

Saves a few bytes of code space and eliminates need for rot_two
bytecode (hence saving RAM and execution time, by a tiny bit).

Escaped quotes are now recognised correctly in REPL when used
inside normal quotes.

Fixes: #1419

Configurable with MICROPY_PY_BUILTINS_SLICE_ATTRS.  Disabled by default.

When creating constant mpz's, the length of the mpz must be exactly how
many digits are used (not allocated) otherwise these numbers are not
compatible with dynamically allocated numbers.

Addresses issue #1448.

4 spaces are added at start of line to match previous indent, and if
previous line ended in colon.

Backspace deletes 4 space if only spaces begin a line.

Configurable via MICROPY_REPL_AUTO_INDENT.  Disabled by default.

Fixes #1435.

Eg 0e0 almost looks like a hex number but in fact is a float.