Move configuration class from matchers to expectations.

- It's general config for rspec-expectations, not
  just for the matchers and fits better there.
- This allows us to cleanup some odd cross-requires
  where rspec/expectations files were requiring
  rspec/matchers files and vice-versa.
- The attribute for this remains on RSpec::Matchers
  because that's where rspec-core looks for it and
  that's been the public API.
Leverage the optimized require logic from rspec-support.

Closes #464
Add changelog entry.

[ci skip]