Allow non-Function callables to be used in count(f, itr)

[tkf]

Currently, all predicate functions are wrapped with

julia/base/reduce.jl

Line 843 in 98e678f

_bool(f::Function) = x->f(x)::Bool

The signature ::Function is overly restrictive because everything is (potentially) callable.

With this patch, following works

julia> struct NonFunctionIsZero end

julia> (::NonFunctionIsZero)(x) = iszero(x)

julia> count(NonFunctionIsZero(), [])
0

Since this worked in Julia 1.0 and 1.4, I'm marking this as a bugfix.

[tkf]

@KristofferC or someone else: I added the backport label since it's a bugfix. Please feel free to remove it if it's inappropriate.