Cleanup surface::elaboration

[Kmeakin]

• Move duplicated logic for synth/checking let terms into synth_let_def and add LetDef struct.
• Replace some uses of let initial_len = self.local_env.len(); ...; self.local_env.truncate(initial_len) with self.with_scope(|this| ...)
• Use new Builder to avoid repeating self.scope.to_scope(...)
• Use for loops to reduce repetition when defining prims
• Move recursion logic in surface::elaboration::order into surface::Term::walk_child_terms

Hopefully this helps with some of the pain mentioned in #488