Use private types in the interface

[yallop]

At present, Uint8.t and Uint16.t are both exposed as equal to int. Consequently, it's easy to build invalid values of those types:

# Usane.Uint8.((257 : t));;
- : Usane.Uint8.t = 257

This PR marks the types Uint8.t and Uint16.t private in the interface so only the functions in the interface, which maintain validity, can be used to create values of those types. It's still easy and free to convert from a Uint8.t or a Uint16.t to an int by coercing:

# let x = Usane.Uint8.of_int 16 ;;
val x : Usane.Uint8.t = 16
# (x :> int);;
- : int = 16

[cfcs]

I think this is a great addition!