Move the directory maker into RSpec::Support

Required before https://github.com/rspec/rspec-core/pull/1565 can be merged.
I'd prefer this to be a positive expectations:

```ruby
Dir.mkdir("tmp")
DirectoryMaker.mkdir_p(dirname)
expect(Dir.exist?(dirname)).to be true
```

This verifies we actually create the remaining directories. Additionally, if an error is raised the spec will still fail.