More info from lambda lifting + allow global captures

[elegios]

This PR has two changes to lambda lifting.

First, we now return a more precise value describing the changes made during the transformation (e.g., we don't depend on iteration order of Names in a Map, we store both the names of captured variables and their corresponding inserted parameters).

Second, there's a new language fragment implementing a slight variation of standard lambda lifting, where we don't insert extra parameters for globally bound variables.

For example, previously

let x = 1 in
let f = lam. x in
... f () ...

would get lambda lifted to something like

let x = 1 in
let f = lam x2. lam. x2 in
... f x () ...

which is useful if you fully want to remove all closures, but redundant if you're fine with letting functions reference top-level values.