There is a form of ApplyPolicy that takes a field and a value type and
returns an ArrayHandle with that value type that can be used directly.
This is implemented with an ArrayHandleMultiplexer.
For some reason, this version of ApplyPolicy magically stopped working.
The compiler, for some reason, grabbed a different overload and then
complained because it tried to use the ValueType as a list. I'm not sure
why this didn't get ignored due to SFINAE. (Maybe a compiler bug.)
At any rate, the problem is fixed by renaming that ApplyPolicy to
ApplyPolicyFieldOfType to make it clear what we are doing and to help
the compiler along.
Rather than do a CastAndCall on all possible field types when calling a
worklet with two fields (where they all typically get cast to the same
type as the primary field), use the new mechanism with
ArrayHandleMultiplexer to create one code path.
Also update the ApplyPolicy to accept the Field type, which is used to
determine any additional storage types to support.
By removing the ability to have multiple CellSets in a DataSet
we can simplify the following things:
- Cell Fields now don't require a CellSet name when being constructed
- Filters don't need to manage what the active cellset is
