Issue Description:

TextOutputStream.write(_:) takes a String as input. This means that the TextOutputStreamable implementation on Substring ends up copying the string, even though the whole point of the protocol is to avoid unnecessary string copies.

In the short term Substring should remove its implementation of TextOutputStreamable so as to not be misleading. In the longer term we should have a solution that allows streaming substrings, because this same issue will affect any type that uses substrings internally and still wants to be streamable.