Handling of GeneratorExit is really peculiar - it subverts normal exception
propagation rules.

Rationale: setting up the stack (state for locals and exceptions) is
really part of the "code", it's the prelude of the function.  For
example, native code adjusts the stack pointer on entry to the function.
Native code doesn't need to know n_state for any other reason.  So
putting the state size in the bytecode prelude is sensible.

It reduced ROM usage on STM by about 30 bytes :)  And makes it easier to
pass information about the bytecode between functions.

For this, needed to implement DELETE_NAME bytecode (because var bound
in except clause is automatically deleted at its end).
http://docs.python.org/3/reference/compound_stmts.html#except :
"When an exception has been assigned using as target, it is cleared at
the end of the except clause."

Based on issues raised during recent review and inconsistency of different
implementations.

This is required to properly handle exceptions across yields.

Remove unnecessary includes. Add includes that improve portability.

Addresses issue #290, and hopefully sets up things to allow generators
throwing exceptions, etc.

Each built-in exception is now a type, with base type BaseException.
C exceptions are created by passing a pointer to the exception type to
make an instance of.  When raising an exception from the VM, an
instance is created automatically if an exception type is raised (as
opposed to an exception instance).

Exception matching (RT_BINARY_OP_EXCEPTION_MATCH) is now proper.

Handling of parse error changed to match new exceptions.

mp_const_type renamed to mp_type_type for consistency.

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.

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.

Also, handle SETUP_FINALLY opcode.

Allows to have nested try blocks with except filters.

TODO: Don't add END_FINALLY's exception re-raise points to traceback.

We still have FAST_[0,1,2] byte codes, but they now just access the
fastn array (before they had special local variables).  It's now
simpler, a bit faster, and uses a bit less stack space (on STM at least,
which is most important).

The only reason now to keep FAST_[0,1,2] byte codes is for compressed
byte code size.

LOAD_METHOD bug was: emitbc did not correctly calculate the amount of
stack usage for a LOAD_METHOD operation.

small int bug was: int was being used to pass small ints, when it should
have been machine_int_t.

This reuses as much str implementation as possible, from this we
can make them more separate as needed.

Also fixes a bug in the for-in-range optimiser.

I hope to remove break and continue byte codes in the future and just
use jump (if possible).

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

Exceptions know source file, line and block name.

Also tidy up some debug printing functions and provide a global
flag to enable/disable them.