Convenience functions for hash maps

These will be reimplemented in the Lean standard library.

def Std.HashMap.mapVal {α : Type u_1} {β : Type u_2} {γ : Type u_3} [BEq α] [Hashable α] (f : α → β → γ) (m : HashMap α β) : HashMap α γ

Apply a function to the values of a hash map.