Takes slice object and sequence length and computes subsequence indexes
for case of slice step=1.

Both return int so far (single-precision float doesn't have enough
bits to represent int32 precisely).

IS_NOT and NOT_IN are now compiled to IS + NOT and IN + NOT, with a new
special NOT bytecode.

Still todo: break/continue from within the finally block itself.

Hopefully this works for other people as well :)

Add mpconfigport.mk file to configure which modules to include into build

Proof of concept, controls "ffi" module as one which requires external
dependencies.

Add exception stack unwind support for RETURN_VALUE.

Conflicts:
	stm/usb.c
	stm/usb.h

Using PendSV interrupt at lowest priority, code can now raise an
exception during an interrupt by calling pendsv_nlr_jump.  The exception
will be raised when all interrupts are finished.  This is used to trap
ctrl-C from the USB VCP to break out of running Python code.

TODO: Decide if we really need separate bytecode for creating functions
with default arguments - we would need same for closures, then there're
keywords arguments too. Having all combinations is a small exponential
explosion, likely we need just 2 cases - simplest (no defaults, no kw),
and full - defaults & kw.

This properly implements return from try/finally block(s).

TODO: Consider if we need to do any value stack unwinding for RETURN_VALUE
case. Intuitively, this is "success" return, so value stack should be in
good shape, and unwinding shouldn't be required.

* Detect VCP line state, based on SET_CONTROL_LINE_STATE request

vm: Introduce structure for exception stack entry, record entry type.

rt_call_function_n_kw did check for integers but not for strings
being called. Added a check so running "a"() won't SIGSEV but
throw an exception.

It's mildly suprising these work without further changes to exception
handling code (the only change required was to handle SETUP_FINALLY).

Also, handle SETUP_FINALLY opcode.