[primitive-types] support deserializing H256 et al from bytes as well as hex strings

[jsdw]

H256 et al serialize to hex strings, presumably to make things a little nicer when working with JSON.

How about we add support to allow them to be deserialized from bytes as well as just hex strings, which is another natural way to represent these things in the serde data model, and allows for better interaction via crates like scale-value (and any other types that prefer to serialize to bytes rather than hex strings), eg: https://github.com/luketchang/scale-value-deser/blob/main/src/main.rs