std::str: remove from_utf8.

This function had type &[u8] -> ~str, i.e. it allocates a string
internally, even though the non-allocating version that take &[u8] ->
&str and ~[u8] -> ~str are all that is necessary in most circumstances.
std::str: s/from_utf8_slice/from_utf8/, to make the basic case shorter.