pdl: create ‘Field’ enum to hold field logic

I’ll slowly be moving over functionality to this new enum and its
variants.

While this might seem like a lot of boiler-plate, this simplifies the
code overall since structs like ‘ScalarField’ know exactly what type
they are. So we’ll have fewer redundant match expressions after this.

Test: atest pdl_tests pdl_inline_tests
Change-Id: Ib9147d4899944e74d2e06bd41e3adc832f89e802