Implementing a type checker for Hull

[rodrigogribeiro]

• This PR introduces a type checker for Hull into Yule.
• Adds test cases for simple Hull programs that are accepted and rejected by the type checker.

[mbenke]

Well done, just some minor issues.

[mbenke]

Actually there is a problem with this weird code:

object Unsafe {
  code {
    function fromWord$unit (x : word) -> unit {
      let result : unit
      assembly {
        result := x
      }
      return result
    }
    function main () -> word {
      fromWord$unit(7)
      return 42
    }
  }
}

in checkAsmStmt:

• mapM_ (lookupVar . show) ns verifies result exists and retrieves its type TUnit, but immediately discards it. Only existence is checked.
• nExpected = length ns = 1 This counts variable names, not slots. result : unit occupies 0 slots, so the expected count should be returnCount TUnit = 0. But the code never looks at the LHS types. - nActual = returnCount TWord = 1.