Implement Mutable trait for StrBuf.
auto merge of #14110 : SSheldon/rust/strbuf_mutable, r=alexcrichton

Despite implementing the Container trait, StrBuf did not implement the Mutable trait and had no clear method; this request implements the clear method of the Mutable trait for StrBuf.

Testing done: added a test and ran `make check` without any failures.