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

The library implements three modules:
* [EndianString](https://github.com/OCamlPro/ocplib-endian/blob/master/src/endianString.cppo.mli) works directly on strings, and provides submodules BigEndian and LittleEndian, with their unsafe counter-parts;
* [EndianBytes](https://github.com/OCamlPro/ocplib-endian/blob/master/src/endianBytes.cppo.mli) works directly on bytes, and provides submodules BigEndian and LittleEndian, with their unsafe counter-parts;
* [EndianBigstring](https://github.com/OCamlPro/ocplib-endian/blob/master/src/endianBigstring.cppo.mli) works on bigstrings (Bigarrays of chars), and provides submodules BigEndian and LittleEndian, with their unsafe counter-parts;
