Virtual package for enabling native int63 support in Base

base-native-int63 is just a virtual OPAM package that determines whether Int63.t in the Base library is exposed as a private int on 64 bit sytems or as an abstract type whose implementation is chosen at runtime.

If you install this package, operations on Base.Int63.t values will be faster as the compiler will be able to generate optimized code. However, you won't be able to use the generated byte-code with js_of_ocaml.