Generalized implementation of FromBase64

Previously, FromBase64 was only implemented on ~[u8] and ~str when
any pointer would do. The implementations of FromBase64 are now
consistent with the implementations of ToBase64.
auto merge of #6796 : sfackler/rust/FromBase64-type-fix, r=bstrie

Previously, FromBase64 was only implemented on ~[u8] and ~str when
any pointer would do. The implementations of FromBase64 are now
consistent with the implementations of ToBase64.