Optimised functions to read and write int16/32/64 from strings and bigarrays

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.