attempt to fix a critical memory leak in Nim's collections

lib/pure/collections/tableimpl.nim

@@ -110,18 +110,24 @@ template hasKeyOrPutImpl(enlarge) {.dirty, immediate.} =
     maybeRehashPutImpl(enlarge)
   else: result = true
 
+proc default[T](t: typedesc[T]): T {.inline.} = discard
+
 template delImpl() {.dirty, immediate.} =
   var hc: Hash
   var i = rawGet(t, key, hc)
   let msk = maxHash(t)
   if i >= 0:
     t.data[i].hcode = 0
+    t.data[i].key = default(type(t.data[i].key))
+    t.data[i].val = default(type(t.data[i].val))
     dec(t.counter)
     block outer:
       while true:         # KnuthV3 Algo6.4R adapted for i=i+1 instead of i=i-1
         var j = i         # The correctness of this depends on (h+1) in nextTry,
         var r = j         # though may be adaptable to other simple sequences.
         t.data[i].hcode = 0              # mark current EMPTY
+        t.data[i].key = default(type(t.data[i].key))
+        t.data[i].val = default(type(t.data[i].val))
         while true:
           i = (i + 1) and msk            # increment mod table size
           if isEmpty(t.data[i].hcode):   # end of collision cluster; So all done
@@ -137,4 +143,6 @@ template delImpl() {.dirty, immediate.} =
 template clearImpl() {.dirty, immediate.} =
   for i in 0 .. <t.data.len:
     t.data[i].hcode = 0
+    t.data[i].key = default(type(t.data[i].key))
+    t.data[i].val = default(type(t.data[i].val))
   t.counter = 0