Separate Wycheproof parser from data

[emillon]

This is a first step towards #7. The pipeline looks like:

1. parse JSON
2. convert to Wycheproof types (full of untyped and optional fields)
3. convert to per-protocol representation
4. run

2 will be extracted to its own library; 4 might be part of a wycheproof-alcotest helper.

cc @pascutto