gc_regions: cleanup & fixes for deallocation (#11920)

* gc_regions: withRegion nows return the modified MemRegion

* gc_regions: make withScratchRegion dealloc correctly

* tests/gc: add tregionleak test

This test checks if memory within regions are freed properly.

tests/gc/tregionleak.nim

@@ -0,0 +1,23 @@
+discard """
+  cmd: '''nim c --gc:regions $file'''
+  output: '''
+finalized
+finalized
+'''
+"""
+
+proc finish(o: RootRef) =
+  echo "finalized"
+
+withScratchRegion:
+  var test: RootRef
+  new(test, finish)
+
+var
+  mr: MemRegion
+  test: RootRef
+
+withRegion(mr):
+  new(test, finish)
+
+deallocAll(mr)