Allow calling high-level functions from Yul blocks

[mbenke]

Simple example

function f() -> word { return 42; }

contract C {
function main() {
  let res: word;
  assembly {
    res := f(42)
  }
  return res;
}

May be more complicated, e.g. f(iszero(iszero(sub(x,1))))

Concept art (see mbenke/callHeaven branch):

specStmt (Asm ys) = do
  ys' <- mapM specYulStmt ys
  pure (Asm ys')
-- specStmt (Asm ys) = Asm ys

specStmt stmt = errors ["specStmt not implemented for: ", show stmt]


specYulStmt :: Yul.YulStmt -> SM Yul.YulStmt
specYulStmt ys = everywhereM (mkM specYulExp) ys

specYulExp :: Yul.YulExp -> SM Yul.YulExp
{-
specYulExp (YCall name args)
    | name `notElem` primYulOps = do
-- result type must be word or () (perhaps the latter only in YExp stmt)
-- arg types must be word (modify typechecker)
-- introduce some variables
-- assign them args (note: Yul probably has righ-to-left eval order)
-- pure (YCall name args')
-}
specYulExp e = pure e

Also need to do something about function renaming, e.g. only rename functions that clash with Yul primOps.