Bindings for adjacent fix-width numerical conversions and flesh out d…

…ocs (#585)

Expose these new primitives to convert between adjacent fix-width types:

- `Int8`
- `Int16`
- `Int32`
- `Int64`

- `Nat8`
- `Nat16`
- `Nat32`
- `Nat64`

And fleshing out the documentation for these modules to be up to
standard with the `Nat` and `Int` modules.

src/Int.mo

@@ -249,6 +249,11 @@ module {
   /// ```motoko include=import
   /// Int.neg(123) // => -123
   /// ```
+  ///
+  /// Note: The reason why this function is defined in this library (in addition
+  /// to the existing `-` operator) is so that you can use it as a function
+  /// value to pass to a higher order function. It is not possible to use `-`
+  /// as a function value at the moment.
   public func neg(x : Int) : Int { -x };
 
   /// Returns the sum of `x` and `y`, `x + y`.