3b7ba4f977 Remove the SliceIndex implementation from hash types (Tobin C. Harding)
Pull request description:
If folk really want to index into a hash they can us `as_byte_array` then index that.
Includes a bump to the version number of `hashes` to `v0.15.0` - this is because otherwise `secp` won't build since we are breaking an API that is used in the current release of secp.
Fix: #3115
ACKs for top commit:
apoelstra:
ACK 3b7ba4f977 successfully ran local tests
Tree-SHA512: 0ba93268cd8133fe683183c5e39ab8b3bf25c15bfa5767d2934d67a5f6a0d2f65f6c9304952315fe8a33abfce488d810a8d28400a28facfb658879ed06acca63