statement compiler will be fixed in a later CL.

The input and output of the expression compiler are now
clearly distinguished.  In the process, I made the individual
expression compilers operate on the compiled form of their
children instead of AST nodes.  As a result, there are now
fewer places where I hand-craft intermediate expression nodes.

The diff looks scarier than it is, mostly because exprCompiler
has been split into the input and output types, resulting in
lots of little renames.

R=rsc
APPROVED=rsc
DELTA=774  (204 added, 199 deleted, 371 changed)
OCL=33851
CL=33851