Recursive let

[ErinvanderVeen]

Resolves #525. Unfortunately, this PR adds "rec" as a keyword, meaning that tests that use "rec" as a shorthand for "record" were changed to use "r" instead.

[yannham]

The implementation looks good, but this PR could benefit from a tad more tests and documentation, see inline comments.

[yannham]

For the free variable tests, I was thinking about adding something like:

#[test]
fn recursive_let() {
    assert!(check_stat_vars(
        "{
          a = let rec b = b + a + h in b,
          b = let rec a = a + b in f (a + 1) z,
          c = let rec foo = b in let rec bar = c in if a.r then [b] else {foo = c}
        }",
        HashMap::from([
            ("a", vec!["a"]),
            ("b", vec!["b"]),
            ("c", vec!["a", "b", "c"])
        ])
    ));
}

in tests/free_var.rs. This checks that the free_var transform correctly handles the case of recursive lets bindings.