add Tuplable

[carllerche]

A proposal for handling tuples. If this looks like a good direction, I will finish it up (docs / tests). I would also probably change Value::Unit -> Value::Tuplable and add Value::Some and Value::None.

[taiki-e]

• I feel using the tuple-specific trait (Tuplable) is good than other known approaches (unnamed Structable that allows "unnamed struct with named fields", Listable that lose some information, etc.).
• For now, I tend to prefer to leave Value::Unit. It can be handled more easily than an empty Tuplable. Also, I feel it is not clear what Tuplable is considered unit. If we have a method that checks for is unit or not, I'm ok with remove Value::Unit.
• I'm in favor of representing Option as Value::Some and Value::None.

[carllerche]

@taiki-e this should be ready for review. I did not fully update valuable-serde to handle all tuples, but did fix it for Unit.

A follow up PR will be needed to add Value::Some(...) and Value::None.