Allow polymorphic types when creating custom bit representations

[rowanG077]

Currently when implementing custom bit representations for any types it needs to be monomorphic. This makes it unergonomic to use. Especially if you make a generic component that relies on some bit representation, enforcing users to derive a bit representation for their concrete version without a way to make that required. It would be much nicer if it is possible to allow quantified types in the annotation and later we instantiate the quantified type with concrete types as used.