fd7f8daeff Move sighash module to crate root (Tobin C. Harding)
Pull request description:
Done as part of the effort to flatten the `util` module.
The `sighash` module can stand alone in the crate root, it provides a discreet set of functionality - the `SighashCache` and associated types.
Marking as high priority because this is part of flattening `util` which is a required step before we start crate smashing.
ACKs for top commit:
Kixunil:
ACK fd7f8daeff
apoelstra:
ACK fd7f8daeff
Tree-SHA512: e812ca903f7dccfa5a06084e23f93f617d016583bdf082d7a36ca8e67e49f1d140a3e138b93939e816861460ff2c04d49d5e37a555dd853dca1c76dbccd910bf