fix all the TODOs from the pull request

* `std.Buffer.print` is removed; use `buffer.outStream().print`
 * `std.fmt.count` returns a `u64`
 * `std.Fifo.print` is removed; use `fifo.outStream().print`
 * `std.fmt.bufPrint` error is renamed from `BufferTooSmall`
   to `NoSpaceLeft` to match `std.os.write`.
 * `std.io.FixedBufferStream.getWritten` returns mutable buffer
   if the buffer is mutable.
4905102901Andrew Kelley committed on 3/13/2020, 4:02:58 PMparent2dd920e
9 files changedLine totals unavailable