Modernize partial extraction

Change "render :partial => 'file'" to "render 'file'", and drop the
funky instance variable handling.
Experimental R-free nav commands

This allows :Emodel instead of :Rmodel, :Smodel instead of :RSmodel, and
so on.  Rationale:

* Provides a pattern that can be followed by plugins for frameworks that
  don't start with an R.
* Alleviates tab completion conflict.  There's currently no :Rconsole or
  :Rgenerator because they would make :Rcontroller and :Rgenerate harder
  to invoke.

This is the start of a very long deprecation cycle.