Addresses issue #377.

Remove unnecessary includes. Add includes that improve portability.

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).

In particular, unix outputs to stderr, to allow to run testsuite against
micropython built with debug output (by redirecting stderr to /dev/null).

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.

Ultimately all static strings should be qstr.  This entry in the type
structure is only used for printing error messages (to tell the type of
the bad argument), and printing objects that don't supply a .print method.

Convert unix open() to such.

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.

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.

Both return int so far (single-precision float doesn't have enough
bits to represent int32 precisely).

Proof of concept, controls "ffi" module as one which requires external
dependencies.

Foreign Function Interface module allows to load native dynamic libraries,
call functions and access variables in them. This makes possible to write
interface modules in pure Python.

This module provides thin wrapper around libffi. ctypes compatibility might
be possible to implement on top of this module (though ctypes allow to call
functions without prototypes, which is not supported by libffi (i.e.
implementation would be inefficient))).

TODO: Check lexer/parse/compile error path for leaks too.

Also, readline uses system malloc, so for symmetry, use the same for
non-readline case.

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

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.

Parser no longer prints an error, but instead returns an exception ID
and message.

Creating of classes (types) and instances is much more like CPython now.
You can use "type('name', (), {...})" to create classes.

Note: only read/write/close methods are defined so far, and class
is not exposed (use open() factory function, as usual).