`Encodable` should take optional context arguments in `encode` and `decode`

[eb8680]

To support advanced use cases like #497 we'll need the Encodable interface to be context-sensitive:

class Encodable[T, U]:
  ...
  @classmethod
  def encode(cls, vl: T, env: Mapping[str, Any]) -> U: ...

  @classmethod
  def decode(cls, vl: U, env: Mapping[str, Any]) -> T: ...

[datvo06]

Hi, would it make sense to have another operation to retrieve context instead? Like in #456 :

@defop
def get_synthesis_context() -> ChainMap[str, Any] | None:
    """Get the current synthesis context for decoding synthesized code."""
    raise NotHandled

and then

 with handler({get_synthesis_context: lambda: template.__context__}):