Retain both the original deferred words and their bases, and expose them with separate methods

[jasmith-hs]

When deferring an expression like {{ deferred.bar }} the deferred.bar is marked as a usedDeferredWord and in the current logic, it's distilled down to its base deferred, but it is helpful to retain the context that deferred.bar is the whole part that got deferred.

So I'm adding a getUsedDeferredBases and getSetDeferredBases, which do what getUsedDeferredWords and getSetDeferredWords did and moved the usages that look for the base to call the new methods instead.

This changes what those methods return for better consistency and this DeferredToken class is marked with @Beta.