Add sizehint! to Dict constructor (#35254)

* Add sizehint to Dict constructor

* Check iterator has length

Author: harryscholes

base/dict.jl

@@ -99,6 +99,10 @@ mutable struct Dict{K,V} <: AbstractDict{K,V}
 end
 function Dict{K,V}(kv) where V where K
     h = Dict{K,V}()
+    chklen = IteratorSize(kv)
+    if (chklen isa HasLength) || (chklen isa HasShape)
+        sizehint!(h, length(kv))
+    end
     for (k,v) in kv
         h[k] = v
     end