We created the `crypto` crate as a container for cryptography modules
with the idea that it may be split out into a separate crate. There is
no reason for users of the lib to know about this module. Also, we have
two `taproot` modules, one in `crypto` and one at the crate root, this
makes for un-ergonomic usage of the lib.
Improve the public API by doing:
- Make the `crypto` module private (`pub(crate)`).
- Re-export `crypto::taproot::Signature` (and `Error`) from
`crate::taproot`
42b07586ac