Remove half of the lambda()s from lang/c.c

c.c is by far the most complicated backend.  This removes about half
the lambdas (by complexity, not by count) from "lang/c.c".
Remove all but one lambda from "lang/c.c"

I'm staging these so "git bisect" will be more efficient because I'm
somewhat convinced that this is going to break things.
Remove lambda.h from lang/c.c

With any luck this won't break everything...
Remove some lambda()s from context.c

All these lambda() calls go directly to stringlist_* calls, so they
are easy to get rid of.