Frost api feedback and minor changes

[nickfarrow]

• add Eq and Hash to Zero NonZero
• synopsis and make docs test pass
• minor doc improvements
• rearrange code - nonce_gen impl SignSession order

Feedback after integration into fedimint branch

• gen_nonce Option of Point does not require type
• add Eq Hash to Zero and NonZero
• implement Default for frost
• expose threshold and n_parties for XOnlyFrostKey
• tweak should not mutate FrostKey
• get SignSession participants double ended iter
• use usize for indexing, use TestRng for prop secrets

[nickfarrow]

Edit: Moved spec commits & discussion into separate PR https://github.com/LLFourn/secp256kfun/pull/119

[nickfarrow]

I think this is ready to (squash &) merge