PPLexer: report prefixed strings/chars, since they are not supported yet
Prepr: check for too many nested includes
Prepr,PPLexer: discard whole lines in false conditionals

When we are after '#if 0', we don't need to parse every token in the
discarded line. We just want to skip the whole line as fast as possible,
while also handling comments.
Prepr: implement partial #if expression parsing
Prepr: expand all values to [u]intmax_t in expressions