Properly handle tombstones when inserting items into a map.

Previously, you could get into a state where a key was present in the
map, but after a tombstone in the probe sequence. If they key was added
again, it stopped at the first tombstone and added it there, resulting
in the key being in the map multiple times.

Fix #373.

test/core/map/reuse_tombstone.wren

@@ -0,0 +1,9 @@
+// Regression test for #373.
+var map = {}
+map[2] = "two"
+map[0] = "zero"
+map.remove(2)
+map[0] = "zero again"
+map.remove(0)
+
+System.print(map.containsKey(0)) // expect: false