Port of @JeffBezanson's OrderedDict

* JuliaLang/julia#10116
* This is largely copy and paste + rename, fixups
* Offers good speed improvements in OrderedDict iteration,
  small improvements elsewhere

src/DataStructures.jl

@@ -63,6 +63,7 @@ module DataStructures
     include("disjoint_set.jl")
     include("heaps.jl")
 
+    include("dictsupport.jl")
     include("hashdict.jl")
     include("ordereddict.jl")
     include("orderedset.jl")

src/dictsupport.jl

@@ -0,0 +1,6 @@
+# support function
+
+# This is defined in Base, but probably not meant for external consumption,
+# so it's redefined here.
+_tablesz(x::Integer) = x < 16 ? 16 : one(x)<<((sizeof(x)<<3)-leading_zeros(x-1))
+

src/hashdict.jl

@@ -2,8 +2,8 @@
 
 import Base: KeyIterator, ValueIterator, haskey, get, getkey, delete!,
              pop!, empty!, filter!, setindex!, getindex, similar,
-             sizehint!, length, filter, isempty, start, next, done,
-             keys, values, _tablesz, skip_deleted, serialize, deserialize
+             sizehint, length, filter, isempty, start, next, done,
+             keys, values, skip_deleted, serialize, deserialize
 
 import Base.Serializer: serialize_type
 const SerState =  Base.Serializer.SerializationState