Ambiguous function name/documentation for int64/uint64 conversion.

[dkuehr]

https://github.com/yallop/ocaml-integers/blob/master/src/unsigned.mli#L93-L94

The function name to_int64 as well as the comment following it don't make clear that the original encoding of the uint64 is preserved. In this sense it could be better to have a distinction between a value conversion (for example when UINT_MAX is added), and a reinterpretation of one type as another (like the pointer cast happening here https://github.com/yallop/ocaml-integers/blob/master/src/unsigned_stubs.c#L197).

For the function name maybe as_int64 could be a good option.

[yallop]

A patch improving the documentation would be very welcome