sign: make to-be-signed function publicly for async signing

[zensh]

In our usage scenarios, two factors require the use of the tbs_data function:

1. Signatures are generated by a trusted remote service and the local service does not hold the private key.
2. The local service may need to use multiple public keys to attempt to verify the signatures.

[zensh]

Hey @daviddrysdale ,
Could you please release a new version? My crate depends on this feature, and I need the new version of coset to be published on https://crates.io/ before I can release my crate there.

[daviddrysdale]

v0.3.8 released.