Minimal Secrets Passing #257

It'd be better if we weren't constantly passing hashes and private keys and descriptors back and forth between WASM and JS. WASM could just keep the secrets in zeroizable memory, and even the hash could be kept in gloo_storage::SessionStorage. This would make the library stateful, so methods will be needed to check whether it's been authenticated. Perhaps this could be done through a fingerprint method; if no seed fingerprint is returned, then the wallet either can't access descriptors, nor does it have a hash.