Reduce consequences of method invalidation for Dict and Set

This aims to reduce disruption when adding methods to
`convert`  and `copymutable`.

Author: timholy

base/dict.jl

@@ -378,6 +378,10 @@ end
 
 function setindex!(h::Dict{K,V}, v0, key::K) where V where K
     v = convert(V, v0)
+    setindex!(h, v, key)
+end
+
+function setindex!(h::Dict{K,V}, v::V, key::K) where V where K
     index = ht_keyindex2!(h, key)
 
     if index > 0

base/set.jl

@@ -67,6 +67,7 @@ delete!(s::Set, x) = (delete!(s.dict, x); s)
 
 copy(s::Set) = copymutable(s)
 
+copymutable(s::Set{T}) where {T} = Set{T}(s)
 # Set is the default mutable fall-back
 copymutable(s::AbstractSet{T}) where {T} = Set{T}(s)