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.

Implementation is duplicated, but tolerate this for now, because there's
no clear idea how to de-dup it.

It's no longer intended to provide just "raw" socket interface, may include
some convenience methods for compatibility with CPython socket - but anyway
just minimal set required to deal with socket client and servers, not wider
network functionality.

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.

If SD card is present on (soft) reset then it's mounted on 1:/ and files
can be openned using, eg, open('1:/test.txt', 'r'), or 'w' for writing.

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.

Implement sys.path support

Fixes Issue #261.

This mirrors CPython behavior and makes possible to run scripts which
import other modules not from script's directory.

If it's not available, "~/.micropython/lib:/usr/lib/micropython" is used
as a fallback.

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.

Initial support for Netduino

Similar to mp_obj_new_tuple().

This also fixes up the IRQ for the PYBOARD4 USERSW
although I was unable to test that functionality.