Use state methods instead of free functions

[brendanzab]

This converts the typing and translation pass functions into methods on their State type. I'm unsure if this is an improvement or not!

The short of it is that instead of:

let (core_term, r#type) = surface_to_core::synth_type(&mut state, &surface_term);

you now write:

let (core_term, r#type) = state.synth_type(&surface_term);