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().

Please don't submit patches with tab indentation!

Test function needs to be called by something else. Test heap size reduced so
its dump fits on a screenful of a typical terminal.

With a nice traceback, helps debugging much better.

If there's malloc and memset, then there's no need for calloc, especially if
we need to implement it ourselves.

Unlike CPython socket, microsocket object already implements stream protocol
(read/write methods), so makefile() just returns object itself. TODO: this
doesn't take care of arguments CPython's makefile() may accept.

Passing keyword arguments to a native function now no longer requires
heap memory.  The kw_args map is created on the stack using the args
array as the table.

This allows keyword maps to be created directly from stack data.

Can't decide which is better for string type, char or byte pointer.
Changing to char removes a few casts.  Really need to do proper unicode.

Linear table at the moment, to eventually be replaced with a hash table
generated by a preprocessor.

Dynamic table is retained so that builtins can be overridden.

Fixes Issue #261.

sys.path is not initialized by rt_init(), that's left for platform-specific
startup code. (For example, bare metal port may have some hardcoded defaults,
and let user change sys.path directly; while port for OS with environment
feature can take path from environment). If it's not explicitly initialized,
modules will be imported only from a current directory.

Similar to mp_obj_new_tuple().