mp_module_obj_t can now be put in ROM.

Configuration of float type is now similar to longint: can now choose
none, float or double as the implementation.

math module has basic math functions.  For STM port, these are not yet
implemented (they are just stub functions).

Some functionality is still missing (eg and, or, bit shift), and some
things are buggy (eg subtract).

Each built-in exception is now a type, with base type BaseException.
C exceptions are created by passing a pointer to the exception type to
make an instance of.  When raising an exception from the VM, an
instance is created automatically if an exception type is raised (as
opposed to an exception instance).

Exception matching (RT_BINARY_OP_EXCEPTION_MATCH) is now proper.

Handling of parse error changed to match new exceptions.

mp_const_type renamed to mp_type_type for consistency.

Convert unix open() to such.

Some tools do not support local/static symbols (one example is GNU ld map file).
Exposing all functions will allow to do detailed size comparisons, etc.

Also, added bunch of statics where they were missing, and replaced few identity
functions with global mp_identity().

Exceptions know source file, line and block name.

Also tidy up some debug printing functions and provide a global
flag to enable/disable them.

Implicit "micropython" module contains (at least) codegeneration decorators.
Make it explicit, so an app could have "import micropython". On MicroPython,
that will be no-op. On CPython, that will give a chance to have a module
with placeholder decorators.

This in particular makes it available for stm port.

mpconfig.h will automatically pull mpconfigport.h.