remove Instruction and replace it with distinct types

[covercash2]

it's currently difficult go from Instruction to Modelfile::<field> because the Instruction enum is an indirection. the initial reasoning for this was to prefer built-in types where it made sense (e.g. from: String) and to avoid unnecessary or premature validation.

but i think it makes better sense to have individual types for these things even if they are just thin wrappers to give more type-safe controls over how these fields are shown, and validation can be added as it becomes necessary