filter on PersistentHashMap inconsistent with Base

[TotalVerb]

The Base version of filter on Dict requires a function that takes two arguments. That's not the case for PersistentHashMap. Arguably, the existing FunctionalCollections behaviour makes more sense; I've filed JuliaLang/julia#17886 over in Base. If Base transitions to one-argument, then nothing needs to be done; otherwise, I feel like PersistentHashMap must be changed too to avoid the surprise factor.