Add XMSS test vectors

[tcoratger]

Depends on the C exports and the Python bindings landing first.

Cross-implementation test vectors, following the layout already used by BLS and KZG: generators under bindings/python/tests/xmss/ that call the spec through the Python bindings and write tests/xmss/<handler>/<case>/data.yaml through dumper.write_case, with one format.md per handler describing the fields.

Handlers worth covering, and the cases that earn their place:

• the raw hash: the RFC 7693 test vectors, the empty input, and 63, 64 and 65 bytes;
• the tweaked hash: one case per tweak type, plus cases showing that changing the type, the position, the index or the public parameter changes the output;
• the message encoding: one admissible encoding, one rejected because a spare bit is set, one rejected because the digits miss 195;
• key generation: epoch 0, a middle epoch, epoch 2^32 - 1; the same seed twice giving the same key; and two different epoch ranges giving different roots;
• signing: valid at the start, middle and end of a range; the same inputs twice giving the same signature; an epoch outside the range;
• verification: a valid signature, then one tampered chain value, one tampered sibling, one tampered randomizer, a wrong epoch, a wrong message, and public keys and signatures of the wrong length.

The negative cases are the point. A test suite that only contains valid signatures passes against an implementation that accepts everything.

One more step that is easy to skip and shouldn't be: run the same inputs through leanEthereum/leanVM once, diff the outputs, and record in format.md which commit of that repository was used. Without it the vectors prove only that this specification agrees with itself.