Can't use keyword as item-fn

[jcf]

There's a pre clause that requires item-fn to be a function, but in the case of maps a keyword may often be appropriate. As a workaround, I can create an anonymous function but it'd be great if a keyword could be passed.

(consistent-hash-fn :db/id identity) ; throws an exception
(consistent-hash-fn #(get % :db/id) identity) ; is okay

I can probably put together a PR this evening if you're open to accepting the change.

[xpe]

@jcf Good idea. Changing the precondition to allow either a function or keyword would work.