This commit adds a sys.implementation.mpy entry when the system supports
importing .mpy files. This entry is a 16-bit integer which encodes two
bytes of information from the header of .mpy files that are supported by
the system being run: the second and third bytes, .mpy version, and flags
and native architecture. This allows determining the supported .mpy file
dynamically by code, and also for the user to find it out by inspecting
this value. It's further possible to dynamically detect if the system
supports importing .mpy files by `hasattr(sys.implementation, 'mpy')`.
