Addresses issue #388.

Implement "yield from"

Partly (very partly!) addresses issue #386.  Most importantly, at the
REPL command line, each invocation does not now lead to increased memory
usage (unless you define a function/lambda).

This reduntant triple is one of the ugliest parts of Python, which they
chickened out to fix in Python3. We really should consider passing just
as single exception instance (without breaking Python-level APIs of course),
but until we do, let's follow CPython layout.

Will be reused in WITH bytecodes.

objgenerator: close(): Throw instance of GeneratorExit (not type).

show_bc now decodes the prelude correctly.  Moved WRITE_FILE stuff from
runtime.c to emitglue.c.

Addresses issue #385.

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

To comply with Python semantics and allow use of mp_obj_is_subclass_fast()
for exception matching.

Only exceptions that are actually used are left prefedined.  Hierarchy
is still there, and removed exceptions are just commented out.

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.

py: Support closures with default args.

Originally, .methods was used for methods in a ROM class, and
locals_dict for methods in a user-created class.  That distinction is
unnecessary, and we can use locals_dict for ROM classes now that we have
ROMable maps.

This removes an entry in the bloated mp_obj_type_t struct, saving a word
for each ROM object and each RAM object.  ROM objects that have a
methods table (now a locals_dict) need an extra word in total (removed
the methods pointer (1 word), no longer need the sentinel (2 words), but
now need an mp_obj_dict_t wrapper (4 words)).  But RAM objects save a
word because they never used the methods entry.

Overall the ROM usage is down by a few hundred bytes, and RAM usage is
down 1 word per user-defined type/class.

There is less code (no need to check 2 tables), and now consistent with
the way ROM modules have their tables initialised.

Efficiency is very close to equivaluent.

Addresses issue #377.

Just to keep things consistent :)

vm: Implement raise statement w/o args (reraising last exception).