Draft out IntDatatype in wiggle-generate (#15)

* Draft out IntDatatype in wiggle-generate

This commit drafts out basic layout for `IntDatatype` structure in
`wiggle`. As it currently stands, an `Int` type is represented as
a one-element tuple struct much like `FlagDatatype`, however, with
this difference that we do not perform any checks on the input
underlying representation since any value for the prescribed type
is legal.

* Finish drafting IntDatatype support in wiggle

This commit adds necessary marshal stubs to properly pass `IntDatatype`
in and out of interface functions. It also adds a basic proptest.

crates/generate/src/names.rs

@@ -86,6 +86,10 @@ impl Names {
         format_ident!("{}", id.as_str().to_shouty_snake_case())
     }
 
+    pub fn int_member(&self, id: &Id) -> Ident {
+        format_ident!("{}", id.as_str().to_shouty_snake_case())
+    }
+
     pub fn struct_member(&self, id: &Id) -> Ident {
         format_ident!("{}", id.as_str().to_snake_case())
     }