listHoldWithKey should have an Incremental interface

[mpickering]

When using listHoldWithKey it is common to want to trigger updates if only a specific part of the map has changed. The function is currently implemented in terms of Incremental but at the last moment the Incremental is turned into a Dynamic and hence the ability for fine-grained dependencies is lost.

Therefore I propose there is at least one interface which returns an Incremental rather than a Dynamic.

mpickering@2c1daaa#diff-79be6de5c8d4ff0b5d5323dd8976a3d7R64

As another point, it seems confusingly named when listHoldWithKey actually returns a map rather than a list.

[cgibbard]

Yeah, it's a "list" in the sense of the document you're meant to be thinking about, but that no longer necessarily makes sense now that this function has moved from reflex-dom to reflex. On the point regarding Incremental, I totally agree.