Remove legacy_modes from test cases
auto merge of #5092 : brson/rust/legacy, r=brson
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
test: De-mut the test suite. rs=demuting