Support `pushfirst!` and `popfirst!` for ordered collections

It would be nice to have these for OrderedDict and OrderedSet. At the moment in Julia 0.7:

julia> using DataStructures

julia> o = OrderedSet([1,2,3])
OrderedSet{Int64}([1, 2, 3])

julia> push!(o, 4)
OrderedSet{Int64}([1, 2, 3, 4])

julia> pop!(o, 4)
4

julia> pushfirst!(o, 0)
ERROR: MethodError: no method matching pushfirst!(::OrderedSet{Int64}, ::Int64)
Closest candidates are:
  pushfirst!(::Any, ::Any, ::Any) at abstractarray.jl:2040
  pushfirst!(::Any, ::Any, ::Any, ::Any...) at abstractarray.jl:2041
  pushfirst!(::Array{T,1}, ::Any) where T at array.jl:1063
  ...
Stacktrace:
 [1] top-level scope at none:0

julia> popfirst!(o)
ERROR: MethodError: no method matching popfirst!(::OrderedSet{Int64})
Closest candidates are:
  popfirst!(::DataStructures.IntSet) at /Users/omus/.julia/packages/DataStructures/eUHI/src/int_set.jl:90
  popfirst!(::BitSet) at bitset.jl:255
  popfirst!(::BitArray{1}) at bitarray.jl:817
  ...
Stacktrace:
 [1] top-level scope at none:0

Start by locating the OrderedDict and OrderedSet implementations and reviewing their existing push! and pop! methods. Check Julia's pushfirst! and popfirst! behavior for ordered collections, then verify that both types support the requested operations with the ordering shown in the issue examples.