Type safety enhancements for Loc

As discussed on the mailing list, it would be advantageous to:

Add a type parameter to the base LocParam trait to allow for safety in the association between typeful LocParam instances such as Title and the type parameter of the Loc.
Remove the overloading of "Param" that currently refers both to LocParam and the value associated with a Loc (potentially provided by a default or rewrite rule)
Add data-aware instances of If, Unless, and Template LocParams
Make LocParam a sealed trait with a well-defined extension point to help the compiler catch possible incomplete match statements.