libsyntax: Make the parser mutable
libsyntax: De-`@mut` `quote_depth` in the parser
libsyntax: De-`@mut` `token` in the parser
libsyntax: De-`@mut` `Parser::span`
libsyntax: De-`@mut` `Parser::last_span`
libsyntax: De-`@mut` `Parser::last_token`
libsyntax: De-`@mut` `Parser::buffer`
libsyntax: De-`@mut` `Parser::buffer_start` and `Parser::buffer_end`
libsyntax: De-`@mut` `Parser::tokens_consumed`
libsyntax: De-`@mut` `Parser::restriction`
libsyntax: De-`@mut` `Parser::obsolete_set`
libsyntax: De-`@mut` `Parser::mod_path_stack`
libsyntax: De-`@mut` `Parser::open_braces`
libsyntax: Remove increasingly inaccurate comment
auto merge of #11226 : pcwalton/rust/mutable-parser, r=alexcrichton

r? @alexcrichton