Pass through literal space values from replacement lists.

Pass through literal space values from replacement lists.

This makes test 15 pass and also dramatically simplifies the lexer.

We were previously using a CONTROL state in the lexer to only emit
SPACE tokens when on text lines. But that's not actually what we
want. We need SPACE tokens in the replacement lists as well. Instead
of a lexer state for this, we now simply set a "space_tokens" flag
whenever we start constructing a pp_tokens list and clear the flag
whenever we see a '#' introducing a directive.

Much cleaner this way.