feat: add context typing so that functions return the type of context passed into them

[jakelorocco]

I hit some very annoying type issues because functions don't pass back the type of context passed into them (according to the type system). In reality, I believe all mellea functions pass back the type of context they were initially given.

We should:

• ensure the assumption that context type of input == context type of output
• raise exceptions if this is not the case to enforce the convention
• think about if exceptions to this convention will be needed (probably just for changing the type of context associated with a given session)

We have debated making this a reality in the before; I think we should just commit to this convention.