Add lazy version of GF.Compile.Compute.Concrete

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.

gf.cabal

@@ -23,6 +23,10 @@ flag server
   Description: Include --server mode
   Default:     True
 
+flag cclazy
+  Description: Switch to lazy compute_concrete (new, experimental)
+  Default: False
+
 library
   build-depends: base >= 4.2 && <5,
                  array,
@@ -181,3 +185,6 @@ executable gf
     other-modules: GF.System.UseSignal
   else
     other-modules: GF.System.NoSignal
+
+  if flag(cclazy)
+    cpp-options: -DCC_LAZY