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.
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