Let expressions are not recursive

[jazullo]

Take the following simple example.

fac :: Int -> Int
fac x = 
  let
    go n = if n > 0 then n * go (n-1) else 1
  in
    go x

This compiles under GHC, but Gibbon yields the following error:

gibbon: L0.Typecheck:
    Unknown function: go in Var "fac"
CallStack (from HasCallStack):
  error, called at src/Gibbon/L0/Typecheck.hs:83:18 in gibbon-0.2-inplace:Gibbon.L0.Typecheck

Evidently let expressions are not typechecked with a recursive context, giving us an unbound identifier go. Supporting the singly recursive case is probably not too troublesome since a recursive context can be made with laziness, a mutually recursive case would probably be too difficult since our let expressions are separated out into single assignments at the AST level.

[rrnewton]

We discussed on the call a strategy to:

• add LetRec to L0 only
• desugar on the way to L1

More details could be fleshed out here.