Implement closure conversion

[jfecher]

Problem

Currently the compiler does no form of closure conversion. Code such as

let x = 2;
let f = |y| x + y;
f(3);

Is represented quite literally after monomorphization leading to functions in SSA referring to a closure's environment from outside of the function, without taking in the environment as a parameter. This leads to odd bugs such as #1088. The new ssa refactor is also more stringent and panics when a closure environment is used this way.

Proposed solution

Going forward we should implement proper closure conversion to solve both these issues. This means translating the above code into the following before/during monomorphisation:

let x = 2;
let f = (x, |x2, y| x2 + y);
f.1(f.0, 3);

Alternatives considered

No response

Additional context

No response

Submission Checklist

• Once I hit submit, I will assign this issue to the Project Board with the appropriate tags.