libsyntax: De-mut the pipe compiler
libsyntax: De-mut the macro parser. rs=demuting
libsyntax: De-mut the parser. rs=demuting
libsyntax: Remove all mutable fields from libsyntax. rs=demuting
etc: Suppress the remaining Valgrind error in rustdoc. rs=burningtree
librustpkg: De-mut librustdoc and librustpkg. rs=demuting
auto merge of #5082 : pcwalton/rust/demut-rustpkg, r=pcwalton