Feature request: setfirst!(::AbstractHeap)

Heaps are particularly useful for keeping track of the smallest (or largest) value in a collection. When being used to keep track of the N smallest values, the root is continually popped, and new elements are put in.

However, replacing the root is currently inefficient, as it entails using both pop! (causing a percolate_down!) and a push! (causing a percolate_up!). Instead, it could be implemented more efficiently like:

function setfirst!(h::AbstractArray, x, o::Ordering=Forward)
    isempty(h) && return push!(h, x)
    @inbounds h[1] = x
    percolate_down!(h, 1, x, o)
    return h
end

It would be nice if functionality like this was exported.

Start by locating the AbstractHeap implementation and the existing pop!, push!, and percolate_down! entry points. Add the requested exported setfirst! functionality for replacing a heap root, including the empty-heap behavior described in the issue, then run the relevant heap tests or add coverage for both cases.