Rewrite TomlTree encoding (#133)

* Rewrite `TomlTree` encoding
* Introduce `TomlTree.WriteTo`

toml.go

@@ -10,13 +10,13 @@ import (
 )
 
 type tomlValue struct {
-	value    interface{}
+	value    interface{} // string, int64, uint64, float64, bool, time.Time, [] of any of this list
 	position Position
 }
 
 // TomlTree is the result of the parsing of a TOML file.
 type TomlTree struct {
-	values   map[string]interface{}
+	values   map[string]interface{} // string -> *tomlValue, *TomlTree, []*TomlTree
 	position Position
 }