Fixes to StreamExts.

- Almost all calls to Stream.Read were broken. These have been patched to all go through ReadBytes which itself has been fixed to function correctly. The key thing to note is that Stream.Read is very much allowed to return less than the requested number of bytes. If this happens and you're not checking the return result, you'll be working with partially initialized arrays and really bad stuff happens when you do that.
- Call CopyTo rather than copying between streams manually.
- Peek and ReadUInt8 have been changed to avoid a pointless array allocation which is significant overhead for such simple calls.
Merge pull request #5509 from RoosterDragon/stream-exts-fixes

Fixes to StreamExts.