Removing Resource::Reference classes

This commit is hopefully less messy than it
first appears, but it's certainly cross-cutting.

The reason for all of this is that we previously only
looked up builtin resource types from outside the parser,
but now that the defined resource types are available globally
via environments, we can push that lookup code to Resource.

Once we do that, however, we have to have environment and
namespace information in every resource.

Here I remove the Resource::Reference classes (except
the AST class), and use Resource instances instead. I
did this because the shared code between the two classes
got incredibly complicated, such that they should have had
a hierarchical relationship disallowed by their constants.
This complexity convinced me just to get rid of References
entirely.

I also make Puppet::Parser::Resource a subclass
of Puppet::Resource.

There are still broken tests in test/, but this was a big
enough commit I wanted to get it in.

Signed-off-by: Luke Kanies <luke@reductivelabs.com>