Added support for ref type hash tables.

This reuses the hash table implementation for objects (and the
associated tests). For efficiency reasons, iterator implementations
are currently adapted rather than calling the TTable code.

tests/table/ptables2.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''true'''
+"""
+
+import tables
+
+proc TestHashIntInt() =
+  var tab = newTable[int,int]()
+  for i in 1..1_000_000:
+    tab[i] = i
+  for i in 1..1_000_000:
+    var x = tab[i]
+    if x != i : echo "not found ", i
+
+proc run1() =         # occupied Memory stays constant, but
+  for i in 1 .. 50:   # aborts at run: 44 on win32 with 3.2GB with out of memory
+    TestHashIntInt()
+
+run1()
+echo "true"