Parser: Add PState and new implementation for parseHead in terms of PState
Parser: Simpler and cleaner implementation for parseHead
Parser: Remove old parseHead implementation
Parser: Add singleOf record to PState and refactor parseHead
Parser: New parseChar implementation
Parser: New parseInt implementation using PState
Parser: No more need for ParserFunc type synonym
Parser: New implementation of parseSpace with PState
Parser: Properly update PState's singleOf record
Parser: Temporary buggy implementation of parseDouble with PState