modstruct: Refactor to support both LE and BE packed structs.
mpconfig.h: Add basic support for target endianness configuration.

A specific target can define either MP_ENDIANNESS_LITTLE or MP_ENDIANNESS_BIG
to 1. Default is MP_ENDIANNESS_LITTLE.

TODO: Autodetect based on compiler predefined macros?
binary: Rename array accessors for clarity.
modstruct: Basic implementation of native struct alignment and types.