Move error_space.rb into space.rb

There's no need for a separate file and there's
going to be a bit of interaction between multiple
spaces at various layers in a stack so having it
in one file will make it easier to see how it relates.
Remove an unnecessary layer of indirection.

* This should be faster (but almost certainly not
  noticably so).
* It was odd to expose these methods off of `RSpec::Mocks`
  given that they are not intended for usage outside
  rspec-mocks.
* We weren't getting any benefit from these except
  for not having to type `.space`.