CPP needs to run after lexing, and integer constant expressions need to be parsed and interpreted for #if. So I'm trying to implement my own since I'm already d

CPP needs to run after lexing, and integer constant expressions need to be parsed and interpreted for #if. So I'm trying to implement my own since I'm already doing lexing/parsing/interpreting. Implementing everything end-to-end also seems like the only way to output decent error messages.