Optimised functions to read and write int16/32/64 from strings,
bytes and bigarrays, based on primitives added in version 4.01.

The library implements three modules:

EndianString works directly on strings, and provides submodules
BigEndian and LittleEndian, with their unsafe counter-parts;
EndianBytes works directly on bytes, and provides submodules
BigEndian and LittleEndian, with their unsafe counter-parts;
EndianBigstring works on bigstrings (Bigarrays of chars),
and provides submodules BigEndian and LittleEndian, with their
unsafe counter-parts;