map.go: Fix Clear() to actually clear the values and keys.

Because groups are using value semantics, the implementation of Clear() was
accidentally clearing a copy of the group instead of the group itself.

Fixes dolthub#20.

map.go

@@ -243,7 +243,8 @@ func (m *Map[K, V]) Clear() {
 	}
 	var k K
 	var v V
-	for _, g := range m.groups {
+	for i := range m.groups {
+		g := &m.groups[i]
 		for i := range g.keys {
 			g.keys[i] = k
 			g.values[i] = v

map_test.go

@@ -216,6 +216,15 @@ func testMapClear[K comparable](t *testing.T, keys []K) {
 		return
 	})
 	assert.Equal(t, 0, calls)
+
+	// Assert that the map was actually cleared...
+	var k K
+	for _, g := range m.groups {
+		for i := range g.keys {
+			assert.Equal(t, k, g.keys[i])
+			assert.Equal(t, 0, g.values[i])
+		}
+	}
 }
 
 func testMapIter[K comparable](t *testing.T, keys []K) {