Previously it was a tuple, which meant it couldn't have sensible
custom instances. Token and TokenType now have Show instances, so we
get more useful output when parsing fails.
The order of initial passes is now:
lex -> preprocess -> structure -> expand-include -> parse
which means that #IFing out structurally-invalid code (like inline VALOF) now
works. This also cleans up the preprocessor code a bit.
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".
