Clean up RAFFormlet interface for safer usage

- Remove -channel from the public interface to <RAFFormlet>. Instead,
only expose the terminal that API consumers are expected to use (for
receiving values entered through the UI and for sending values to update
the UI). This resolves the issue of it being really confusing which
terminal to use.

- Move valueTransformer to RAFPrimitiveFormlet, since it has no effect
on RAFCompoundFormlet!
Added -totalDataSignal, analogous to -validationSignal but unvalidated

This merges both terminals of the form's channel (so that you can have
an always-up-to-date signal of the form's data). This is analogous to
the -validationSignal, but it is unvalidated.