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)`

tomltree_write_test.go

@@ -146,7 +146,7 @@ func TestTomlTreeWriteToMapSimple(t *testing.T) {
 func TestTomlTreeWriteToInvalidTreeSimpleValue(t *testing.T) {
 	tree := TomlTree{values: map[string]interface{}{"foo": int8(1)}}
 	_, err := tree.ToTomlString()
-	assertErrorString(t, "invalid key type at foo: int8", err)
+	assertErrorString(t, "invalid value type at foo: int8", err)
 }
 
 func TestTomlTreeWriteToInvalidTreeTomlValue(t *testing.T) {