Sweep: Implement compilation for rnix LetIn expressions

[theoparis]

A nix expression containing let in looks something like this

let
  x = 1;
  y = 2;
in x + y

There should be a function in lib.rs that compiles a individual expression which matches on the Expr type and looks for LetIn nodes and Literal's. It should utilize cranelift

[sweep-ai[bot]]

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