Split up derive and make test assertions better

[KodrAus]

This PR works on our sval_test library to make it produce more useful messages when implementations of Value fail. A Value shouldn't fail itself, similarly to the expectation on Debug and Display, but if it does we should make some effort to communicate why.

I'm also splitting up sval_derive so that we can add extensions to it, like #[flatten], that might make use of other libraries like sval_buffer or sval_fmt.