mp_lexer_t type is exposed, mp_token_t type is removed, and simple lexer
functions (like checking current token kind) are now inlined.

This saves 784 bytes ROM on 32-bit unix, 348 bytes on stmhal, and 460
bytes on bare-arm.  It also saves a tiny bit of RAM since mp_lexer_t
is a bit smaller.  Also will run a bit more efficiently.

This makes it easier to re-use readline.c and pyexec.c from stmhal in
other ports.

Improvements are:

2 ctrl-C's are now needed to truly kill running script on pyboard, so
make CDC interface allow multiple ctrl-C's through at once (ie sending
b'\x03\x03' to pyboard now counts as 2 ctrl-C's).

ctrl-C in friendly-repl can now stop multi-line input.

In raw-repl mode, use ctrl-D to indicate end of running script, and also
end of any error message.  Thus, output of raw-repl is always at least 2
ctrl-D's and it's much easier to parse.

pyboard.py is now a bit faster, handles exceptions from pyboard better
(prints them and exits with exit code 1), prints out the pyboard output
while the script is running (instead of waiting till the end), and
allows to follow the output of a previous script when run with no
arguments.

There is no need, since we don't (currently) use the value.

Found these by compiling stmhal with mp_uint_t of type uint32_t instead
of unsigned int.  This actually makes a difference to the code, but just
a curiosity.

mp_parse_node_free now frees the memory associated with non-interned
strings.  And the parser calls mp_parse_node_free when discarding a
non-used node (such as a doc string).

Also, the compiler now frees the parse tree explicitly just before it
exits (as opposed to relying on the caller to do this).

Addresses issue #708 as best we can.

Recent changes to builtin print meant that print was printing to the
mp_sys_stdout_obj, which was sending data raw to the USB CDC device.
The data should be cooked so that \n turns into \r\n.

Refactored some stmhal files which are shared with teensy.

Addresses issue #558, but it's likely that other out-of-memory errors
could crash the pyboard.  Reason is that qstrs use m_new and can raise
an exception within the parser.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

Make include dependencies neater, and adheres to the coding convention
that headers should not include headers.

Any generated headers go in $(BUILD)/genhdr/, and are #included as
'genhdr/xxx.h'.

Internal flash used for the filesystem is now written (from the cache)
only after a 5s delay, or when a file is closed, or when the drive is
unmounted from the host.  This delay means that multiple writes can
accumulate in the cache, and leads to less writes to the flash, making
it last longer.

It's implemented by a high-priority interrupt that takes care of flash
erase and write, and flushing the cache.

This is still only an interim solution for the flash filesystem.  It
eventually needs to be replaced with something that uses less RAM for
the cache, something that can use more of the flash, and something that
does proper wear levelling.

Full CPython compatibility with this requires actually parsing the
input so far collected, and if it fails parsing due to lack of tokens,
then continue collecting input.  It's not worth doing it this way.  Not
having compatibility at this level does not hurt the goals of Micro
Python.

Also add command line option to unix port to select emitter.

Mostly just a global search and replace.  Except rt_is_true which
becomes mp_obj_is_true.

Still would like to tidy up some of the names, but this will do for now.

Adds readline_init() to clear readline history on soft reset.  Addresses
issue #387.

New USB HAL is quite a bit improved over previous one.  Now has better
callbacks and flow control.

REPL over USB CDC now works as before, except for soft-reset (since USB
driver uses malloc...).

Stack layout is different when -g used, so need to handle the pendsv
jump differently.  Addresses Issue #315.

Raw REPL mode is activated by CTRL-A, and allows you to more easily
communicate with the pyboard over the serial channel.  In particular,
automated communication using, eg, pyserial on the PC side.