Rework tree from map (#139)

* Make TreeFromMap reflect to construct tree
* Fix wording of invalid value type in writeTo

Fixes #138, #139, #134 

⚠️ TreeFromMap signature changed to `TreeFromMap(map[string]interface{}) (*TomlTree, error)`

parser_test.go

@@ -662,10 +662,11 @@ func TestTomlValueStringRepresentation(t *testing.T) {
 }
 
 func TestToStringMapStringString(t *testing.T) {
-	in := map[string]interface{}{"m": TreeFromMap(map[string]interface{}{
-		"v": &tomlValue{"abc", Position{0, 0}}})}
+	tree, err := TreeFromMap(map[string]interface{}{"m": map[string]interface{}{"v": "abc"}})
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
 	want := "\n[m]\n  v = \"abc\"\n"
-	tree := TreeFromMap(in)
 	got := tree.String()
 
 	if got != want {