Improve performance of sizehint! on BinaryHeap

[ameligrana]

Noticed that sizehint! was actually worsening a lot of the performance of some of my heaps, with this change it is much faster:

e.g. with

using DataStructures, BenchmarkTools
function with_sizehint(T, n)
    value = BinaryHeap(Base.Order.ForwardOrdering(), T[])
    sizehint!(value, n)
end

before:

julia> @btime with_sizehint(Int, 1);
  861.250 ns (5 allocations: 256 bytes)

after:

julia> @btime with_sizehint(Int, 1);
  64.790 ns (4 allocations: 224 bytes)

[ameligrana]

Oh actually 1.6 doesn't support @inline at callsite, so I will inline it in the definition

[oxinabox]

Great, I have back ported this also to 0.18.20
should be available in 10 minutes or so