* 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.