The existing constant-folding code in the parser is still there, since it needs
to know whether things are constant, and A.Dimension expects an Int.
However, this pass is useful because it does a better job of constant folding
than the parser would on its own: it can fold subexpressions of expressions
that are as a whole not constant.
This implements #DEFINE, #UNDEF, #IF, #ELSE and #ENDIF, macro expansion with
##, and TRUE, FALSE, AND, OR, NOT and DEFINED within #IF expressions, with the
same semantics as occ21.
The macro COMPILER.TOCK is always defined by default, so you can now say things
like "#IF NOT DEFINED (COMPILER.TOCK) ... #ENDIF".
