The three classes I2C, SPI and USART now have a fairly uniform (Python)
API.  All are constructed, initialised and deinitialised in the same
way.  They can have most of their parameters set, using keyword arguments.
All have send and recv (although slightly different with I2C requiring an
address in master mode).  recv can do inplace receiving (ie store the
data in a previously-created bytearray).

It's just polling mode at the moment, but interrupt and DMA would be
nice to add.

The HAL handles for the I2C/SPI objects are rather large, so we don't
want to unnecessarily include them.

Also, checked that both bytearray and array.array actually support generic
(a-la list) slice assignment and deletion. Added TODOs.

Merge branch 'fix-netduino-i2c-spi' of github.com:dhylands/micropython into dhylands-fix-netduino-i2c-spi

No logic changes, just coding style to make it easy to read.

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.