-Introduce PersistentParserState to represent state persistent among multiple parsing passes.
The advantage is that PersistentParserState is independent of a particular Parser or Lexer object.
-Use PersistentParserState to keep information about delayed function body parsing and eliminate parser-specific
state from the AST (ParserTokenRange).
-Introduce DelayedParsingCallbacks to abstract out of the parser the logic about which functions should be delayed
or skipped.
Many thanks to Dmitri for his valuable feedback!
Swift SVN r6580
