Make IntSet a generic ordinal set OrdSet[A] (#15564)

* Make IntSet an ordinal set OrdSet[A: Ordinal]

Backward compatibility with IntSet is maintained.
IntSet is an alias for OrdSet[int]

* move ordsets to new file, intsets exports it
* ordset, move to lib/std folder

* Fix `$` for ordsets and test cleanup
* Fix ordsets compilation in doc example
* Rename ordsets to packedsets

tests/stdlib/tintsets.nim

@@ -1,65 +0,0 @@
-import intsets
-import std/sets
-
-from sequtils import toSeq
-from algorithm import sorted
-
-proc sortedPairs[T](t: T): auto = toSeq(t.pairs).sorted
-template sortedItems(t: untyped): untyped = sorted(toSeq(t))
-
-block: # we use HashSet as groundtruth, it's well tested elsewhere
-  template testDel(t, t0) =
-
-    template checkEquals() =
-      doAssert t.len == t0.len
-      for k in t0:
-        doAssert k in t
-      for k in t:
-        doAssert k in t0
-
-      doAssert sortedItems(t) == sortedItems(t0)
-
-    template incl2(i) =
-      t.incl i
-      t0.incl i
-
-    template excl2(i) =
-      t.excl i
-      t0.excl i
-
-    block:
-      var expected: seq[int]
-      let n = 100
-      let n2 = n*2
-      for i in 0..<n:
-        incl2(i)
-      checkEquals()
-      for i in 0..<n:
-        if i mod 3 == 0:
-          if i < n div 2:
-            excl2(i)
-          else:
-            t0.excl i
-            doAssert i in t
-            doAssert not t.missingOrExcl(i)
-
-      checkEquals()
-      for i in n..<n2:
-        incl2(i)
-      checkEquals()
-      for i in 0..<n2:
-        if i mod 7 == 0:
-          excl2(i)
-      checkEquals()
-
-      # notin check
-      for i in 0..<t.len:
-        if i mod 7 == 0:
-          doAssert i notin t0
-          doAssert i notin t
-          # issue #13505
-          doAssert t.missingOrExcl(i)
-
-  var t: IntSet
-  var t0: HashSet[int]
-  testDel(t, t0)