Expanded the Rails spec.
Re-generated the Rails definitions.
Fixed incorrect parenthesis in the defs template.
Re-introduced autoloading of definitions.

Requiring users to manually load definitions isn't going to work as I don't
expect users to do so. Eager loading everything, even when not used, also adds
too much overhead (around 200 ms alone for the standard library). This new
setup needs some extra cleanup work but I'll be doing that in the following
commits.