Add and expose typed PreconditionerVariant

[leostimpfle]

This PR addresses the "variant" part of #236. I'll address the "build times" in a second PR.

I've made three key design choices (which can be debated):

• Introduced an exhaustive Rust PreconditionerVariant enum representing the algorithm of a built preconditioner: Additive or Diagonal.
• This is kept separate from the non-exhaustive PreconditionerConfig: the config describes a construction request while the variant describes what an existing Preconditioner contains.
• Kept Preconditioner::variant_name() for backward compatibility, but deprecated it in favour of the typed PreconditionerVariant. (This is mostly to avoid a having to bump the version to 0.4.0.)