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.
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).