Add an `alloc` feature & a generic `Value` type

[zeenix]

Creating a separate issue for this from the points I made here and here. To summarize, a generic Value type would be super useful to have but it will require allocations and hence I understand why it's currently a non-goal. However, to make the most out of Rust enums (after that is added), people will have to enable alloc feature of serde and IMO that sort of beats the point of resisting addition of an alloc feature to this crate.

So I'm suggesting addition of an optional alloc feature along with a generic Value type. I would most likely be able to provide a PR for this, if maintainers show interest.