Add support for finding load time warnings

Extraction of the supporting code from rspec/rspec-core#1654.
I'm still not convinced this is needed or really even benefits us.  Users aren't going to require this file unless they are trying to do something non-standard that they already know we don't officially support.  If we're going to guard any loading of stdlib in this file then that suggest we need to do that in all spec files and supporting files.

There are also already other instances of libraries being required from some of the `rspec-support/lib/rspec/support/spec` files:

* https://github.com/rspec/rspec-support/blob/fcce4a537e3ea2c9278a026434db4928bfd37977/lib/rspec/support/spec/stderr_splitter.rb#L1
* https://github.com/rspec/rspec-support/blob/fcce4a537e3ea2c9278a026434db4928bfd37977/lib/rspec/support/spec/with_isolated_directory.rb#L1
* https://github.com/rspec/rspec-support/blob/fcce4a537e3ea2c9278a026434db4928bfd37977/lib/rspec/support/spec.rb#L45

...so this is already a more general problem than in this one file, so if we were to address it I'd want to address it more holistically but I don't see it actually being a problem in practice.