3 emitter functions are needed only for emitcpy, and so we can #if them
out when compiling with emitcpy support.

Also remove unused SETUP_LOOP bytecode.

Closed over variables are now passed on the stack, instead of creating a
tuple and passing that.  This way memory for the closed over variables
can be allocated within the closure object itself.  See issue #510 for
background.

Avoids pointer-to-field garbage collection issue. Fixes #510.

There were typos, various rounding errors trying to do concurrent counting
in bytes vs blocks, complex conditional paths, superfluous variables, etc.,
etc., all leading to obscure segfaults.

Useful as callback data, etc.

Also some updates to compile with latest changes to core py.

These are to assist in writing native C functions that take positional
and keyword arguments.  mp_arg_check_num is for just checking the
number of arguments is correct.  mp_arg_parse_all is for parsing
positional and keyword arguments with default values.

'p' in struct module is "pascal string". 'P' is void*.

It's Light Emitting Diode, so should be LED.

Main reason for expanding buffer protocol API was to support writes to a
buffer in ADC module (see read_timed).  With this change you can now
create an array of arbitrary type and ADC.read_timed will store into
that array in the correct format (byte, int, float).  I wonder though if
all these changes were really worth it to support just this function.
Hopefully this enhanced buffer protocol API (with typecode specified)
will be used elsewhere.