add deleteat! for OrderedDict

[bjarthur]

i can add tests if you'd like to merge

[Tokazama]

I'm not sure if we yet to converge on a consensus on how users should interface with the index here (JuliaCollections/DataStructures.jl#180 (comment)).

[bjarthur]

indexing seems to work as (at least i would) expect(ed) now. should i then add tests to this PR so it can be merged?

julia> using OrderedCollections

julia> function testindexing()
           s = OrderedSet([i for i = 1:12])
           delete!(s, 7)
           prerh = s[7]
           OrderedCollections.rehash!(s.dict)     # see #392 for the OrderedCollections name
           postrh = s[7]
           return (prerh, postrh)
       end
testindexing (generic function with 1 method)

julia> testindexing()
(8, 8)