Box BindingData output type separate from minor types

[marner2]

COMBINED WITH #508

Conflicts with #506 because of the moved createBinding function.

Box the output type T of BindingData separately from boxMinorTypes methods (and Binding.fs helpers). Currently, all functions in Binding.fs box it together, but this change allows for other constructs that don't box it.

[LyndonGingerich]

From #470 (comment):

I am not convinced that the changes in this file are needed yet. For example, BindingData.OneWaySeq.boxMinorTypes boxes 'a while Helpers.createBinding boxes 'aCollection. Can you change this PR so that aCollection is boxed in each binding type's boxMinorTypes function?

-- @TysonMN

When we add the 't type parameter to BindingData, we are promoting types like 'aCollection from a "minor" type to a major type. Subsequently (to maintain backwards compatibility) Helpers.createBinding boxes one of the major types of BindingData (ie, 't) so that we don't break existing code.

In one of the next PRs, we will be adding something like Helpers.createBindingT that doesn't box the major type, and then use it in the new StaticHelper class.

-- @marner2

[TysonMN]

This is less efficient and convincingly correct for you static code, but what about (just to start) you unbox 't in the future PR instead of merging this PR?

[marner2]

I think you mean just merging #511 first? I actually split them apart so you can merge that one first.

In the end though, we need this type passed all the way through, otherwise we'll be stuck putting type annotations on the view model properties.

[marner2]

@TysonMN I am starting to do (approximately) this in #522.

[marner2]

Closed, combined with #508 to actually implement the forward-facing feature in totality.