str: Implement Container for ~str, @str and Mutable for ~str

~str and @str need separate implementations for use in generic
functions, where it will not automatically use the impl on &str.
rm obsolete no-op lints
auto merge of #7932 : blake2-ppc/rust/str-clear, r=huonw

~str and @str need separate implementations for use in generic
functions, where it will not automatically use the impl on &str.

fixes issue #7900
Merge pull request #7936 from thestinger/cleanup

rm obsolete no-op lints
Merge branch 'master' of github.com:mozilla/rust