Add support for math integers

[W95Psp]

This PR adds support for math integers. Will fix #607.

This PR:

• introduces a Int type (for math integers) that implements common operations;
• translate Int as primitive int in F*, and same thing for methods on Int;
• introduces a Specialize phase that "monomorphize" operations (e.g. std's add becomes a specific function);
• provides a int macro for math int literals;
• adds tests for math ints and their operations.

[franziskuskiefer]

Thanks, a couple notes.