::std::cast::transmute -> ::std::mem::transmute

As per https://github.com/mozilla/rust/commit/f94d671bfae5d8e9a4a4add310b1c40af0ab62a6
Merge pull request #4 from alexanderchr/master

::std::cast::transmute -> ::std::mem::transmute
~str -> StrBuf
StrBuf -> String