pdl: Use num-traits in canonical tests

The FromPrimitive and ToPrimitive traits allow us to be generic over
the value types in the canonical tests.

This is helpful when we introduce support for enums: there we will
need to convert to/from non-integer types and we will have to do so
without knowing the actual type when we generate the code. The traits
allow us to do so in a generic way.

Test: atest pdl_rust_generator_tests_{le,be}
Change-Id: I11542ff298d924a0f6d0492445267c9efe37ace8