pdl: Specialize Field::Fixed into FixedScalar and FixedEnum

There existed an implicit assumption that width, value and
enum_id, tag_id were mutually exclusive in the Field::Fixed
case.
This change makes the assumption explicit by modiyfing
the AST to add specialized cases.

Test: cargo build && cargo test
Change-Id: Ib51f61ad2cd39f8686ad9d94a6699cf2b3b318e7