Move operator matchers into `built_in` directory.

Lazy loading it with autoloading is nice since
it won't be even be used when folks use only
the `expect` syntax.
Remove expression generator logic.

Given we are recommending the use of `expect`
in RSpec 3, no need to include `should` in
error messages.
Prefer `module_function` to `extend self`/`def self.`.
Move specs into files where they better belong.