Parser: Drop EX_SINGLE, de-dent expression.

EX_SINGLE never did anything except keep EX_CONDITION and EX_SAVE_AST from
working unless it was set. I think I did this intentionally a long time ago.
Something about it being faster to check that one flag before checking others
or maybe I actually wanted to make expression multi-line and forgot.

Whatever the reason, statement is now multi-line as of the last commit, and I
probably won't ever make expression multi-line.