Use a specialized string interner to reduce the need for owned strings

&str can be turned into @~str on demand, using to_owned(), so for
strings, we can create a specialized interner that accepts &str for
intern() and find() but stores and returns @~str.
auto merge of #6356 : dotdash/rust/strinterner, r=pcwalton

&str can be turned into @~str on demand, using to_owned(), so for
strings, we can create a specialized interner that accepts &str for
intern() and find() but stores and returns @~str.