Add compute proc for SharedTable (#5385)

lib/pure/collections/tableimpl.nim

@@ -120,9 +120,7 @@ template default[T](t: typedesc[T]): T =
   var v: T
   v
 
-template delImpl() {.dirty.} =
-  var hc: Hash
-  var i = rawGet(t, key, hc)
+template delImplIdx(t, i) =
   let msk = maxHash(t)
   if i >= 0:
     dec(t.counter)
@@ -145,6 +143,11 @@ template delImpl() {.dirty.} =
         else:
           shallowCopy(t.data[j], t.data[i]) # data[j] will be marked EMPTY next loop
 
+template delImpl() {.dirty.} =
+  var hc: Hash
+  var i = rawGet(t, key, hc)
+  delImplIdx(t, i)
+
 template clearImpl() {.dirty.} =
   for i in 0 .. <t.data.len:
     when compiles(t.data[i].hcode): # CountTable records don't contain a hcode