Add #to_io and #reopen methods to `StdErrSplitter`

These changes are required to support capturing the `$stderr` stream in a tempfile rather than a `StringIO`. See https://github.com/rspec/rspec-expectations/pull/700 for the rationale.
:+1:

I always assumed `#to_io` was reserved for file descriptor-backed IO objects. Can't remember where I got that idea from though.