Add into_* methods to Value

[dodomorandi]

I propose the implementation of into_ helpers for Value. I opted for some design choices, which can reconsider without problem:

• Return values are Results, and the Err type is Self. In this way the value is not lost if the conversion fails. Maybe it is a bit less ergonomic than returning Option, but it should be slightly more powerful.
• I used Self in Err, but Value in Ok for nested things like Array and Map. This is only to avoid types like Return<Vec<Self>, Self>, which can be slightly confusing.
• I decided to implement into_ methods for Copy types as well. I made it for consistency, but to be honest I am a bit unsure.
• I omitted into_null, which would have the following signature:

  fn into_null(self) -> Result<(), Self>

  I think that this would be pretty useless.

Let me know what do you think, I will be happy to receive feedback, discuss and change things in order to expose a nice API.