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.

Required to build bare-arm with debugging.
Signed-off-by: Sven Wegener <sven.wegener@stealer.net>

This makes open() and _io.FileIO() more CPython compliant.
The mode kwarg is fully iplemented.
The encoding kwarg is allowed but not implemented; mainly to allow
the tests to specify encoding for CPython, see #874

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.

qstr_init is always called exactly before mp_init, so makes sense to
just have mp_init call it.  Similarly with
mp_init_emergency_exception_buf.  Doing this makes the ports simpler and
less error prone (ie they can no longer forget to call these).