Preliminary new shell feature: cc -trace.

You can now do things like 

	cc -trace mkV "debug"

to see a trace of all opers with their arguments and results during the
computation of mkV "debug".

src/compiler/GF/Grammar/Predef.hs

@@ -61,6 +61,7 @@ cRead = identS "read"
 cToStr = identS "toStr"
 cMapStr = identS "mapStr"
 cError = identS "error"
+cTrace = identS "trace"
 
 -- * Hacks: dummy identifiers used in various places.
 -- Not very nice!