This also allows the parameter rec to be removed from function computeTermOpt.
(The change is made in GF.Compile.Compute.ConcreteLazy, but not in
GF.Compile.Compute.ConcreteStrict.)
This patch adds GF.Compile.Compute.ConcreteLazy, which replaces the Err monad
with the Identity monad. While the Err monad makes the interpreter
(hyper)strict, the Identity monad let's the interpreter inherit Haskell's
laziness. This can give big speedups: from 50s to 1s in one example,
from ~4 minutes to ~2 minutes for the RGL.
This is still experimental and might be buggy, so it is off by default.
You can turn it on by configuring with the -fcclazy flag, e.g.
cabal configure -fcclazy
Let me know if anything breaks.
