updates encoding_instructions a type directed operation separate from Encodable

[kiranandcode]

Closes #516

[eb8680]

Why is this separate from Encodable? I thought it was supposed to be a method or Operation attached to Encodable and take a context argument?:

class Encodable[T](EncodableAs[T, type]):
  ...
  @Operation.define  # or only define an `Operation` for specific `Encodable` subtypes?
  @classmethod
  def encoding_instructions(cls, env: Mapping[str, Any]) -> Sequence[str]: ...

[kiranandcode]

Maybe a miscommunication? I thought @jfeser last suggestion was to replace the method with a standalone type directed function, alongside making it lexically sensitive

If thats not necessary then I'll update the PR to just make it a defop and add lexical context as a parameter