Remove unnecessary bounds check from mp_seq_get_fast_slice_indexes.

At this point, start will be >= 0, so checking if stop < 0 is redundant with
checking if start > stop a few lines later.
Merge pull request #675 from Rosuav/seq_simplify

Remove unnecessary bounds check from mp_seq_get_fast_slice_indexes.
py: Make import recognize and load native modules (.pyd or .so files)

Simply check if a .pyd or .so file exists on the Python search path
(if a .py file if not found) and call a function to get a module from it
msvc: Implement LoadDynLib
msvc: Make part of uPy dynamically loadable via import lib

Exports some core functions using exports.def, export constants/data using the
typical dllexport (exporting data using .def file yields multiple copies of data).
Create an import lib next to micropython.exe whcih can be used by modules.
msvc: Add property sheet for building native modules, update README