Async mut-less Signer trait

[ximon18]

Async rationale: the synchronous or asynchronous nature of signer behaviour is implementation specific, e.g. it might be an HSM contacted over a network via the KMIP TCP+TLS protocol which involves waiting which ideally wouldn't be blocking.

Mut-less rationale: the granularity of locking is signer implementation dependent, taking the KMIP HSM example again the HSM will impose its own locking, from a client perspective the protocol is stateless and permits me to make multiple key creation or other requests in parallel but with &mut in the Signer trait fns the caller is currently forced to lock all access to the Signer when using &mut fn.