Having a private `pretty_print` method causes problems.

When rspec-expectations tries to diff the object in
a failure message, it blows up when the `pp` library
tries to print it.
Refactor: store const mutators in mock space.

This unifies global storage of modified things in one place.
Re-work spec file a bit.

- Make it more general (it's not just about stubbing).
- Remove unused lines.
- Improve doc output (`include_examples` does not
  create a nested group).
Don't allow const stubbing/hiding from before(:all).
Refactor space tracking so it uses a stack and allows nesting.
Add RSpec::Mocks.with_temporary_scope.

Closes #240.