RUSTSEC-2020-0105 Update unsound DrainFilter and RString::retain

Affected versions of this crate contained code from the Rust standard library that contained soundness bugs rust-lang/rust60977 double drop & rust-lang/rust78498 create invalid utf-8 string. The flaw was corrected in v0.9.1 by making a similar fix to the one made in the Rust standard library...