Not all functions implemented.  Not enabled on pyboard.

Disabled by default.  Enabled in unix port.

Enabled by MICROPY_ENABLE_PROPERTY.

This is to reduce ROM usage.  stream_p is used in file and socket types
only (at the moment), so seems a good idea to make the protocol
functions a pointer instead of the actual structure.

It saves 308 bytes of ROM in the stmhal/ port, 928 in unix/.

Finishes addressing issue #424.

In the end this was a very neat refactor that now makes things a lot
more consistent across the py code base.  It allowed some
simplifications in certain places, now that everything is a dict object.

Also converted builtins tables to dictionaries.  This will be useful
when we need to turn builtins into a proper module.

GC support for "unix" port was implemented some time ago, but still not
enabled.

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).

This change allows to build unix version without libreadline installed.

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

Convert unix open() to such.

Exceptions know source file, line and block name.

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

Byte code has a map from byte-code offset to source-code line number,
used to give better error messages.

This in particular makes it available for stm port.

mpconfig.h will automatically pull mpconfigport.h.

A big change.  Micro Python objects are allocated as individual structs
with the first element being a pointer to the type information (which
is itself an object).  This scheme follows CPython.  Much more flexible,
not necessarily slower, uses same heap memory, and can allocate objects
statically.

Also change name prefix, from py_ to mp_ (mp for Micro Python).