pdl: Add support for enums

This adds support for parsing and serializing enum fields.

The canonical test vectors work.

Tests have been added to the canonical PDL files: they demonstrate
that we can generate valid Rust code. A test was added to show that we
need “#[repr(u64)]” for an enum with a maximum discriminant.

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: Id65bf831ddde8ade9780f89096842ef9516cf4c3