rustc::back::link: redo symbol mangling

Handle more characters that appear in types, most notably <>): were
missing. Also the new scheme takes care that no two different input
strings result in the same mangled string, which was not the case before.

Fixes #6921
auto merge of #6956 : Blei/rust/fix-symbol-mangling, r=catamorphism

Handle more characters that appear in types, most notably <>): were
missing. Also the new scheme takes care that no two different input
strings result in the same mangled string, which was not the case before.

Fixes #6921