Add Value type, with serde and scale impls

[jsdw]

• The Value type is the simplest shape possible to be able to represent the data for arbitrary types generated from scale-info (it does not care about any type information; only that you don't lose any precision encoding and decoding things from/to Values given a specific type to encode/decode to/from).
• Encoding values is fairly lenient w.r.t the shape of the value type; if the Value can be encoded into the type losslessly we attempt to allow it.
• Serde impls exist to allow the Value type to be transformed from/to some serial format and static types (though at the time of writing, going from a static type to a scale value hasn't been implemented yet) (edit: I did add this).

The main use case for this crate offhand will be allowing subxt to work with dynamic values more, and be able to provide back a representation for scale encoded bytes that a static type may not exist for.

[lexnv]

Suggested change

	// This file is part of subxt.
	// This file is part of scale-value.

[jsdw]

Ooh good spot; need to go around and change these copyright headers now :)

[ascjones]

LGTM!