4ead0adcb5 Add blanket impl of io traits for `&mut T` (Martin Habovstiak)
Pull request description:
The impl wasn't previously available because we thought we can do a blanket impl for `std` traits. We've removed the `std` blanket impl when we realized it's broken but forgot to add the `&mut T` impls. This adds them.
Note: this is in part an experiment to see if this is API breaking. I suspect it might be.
ACKs for top commit:
apoelstra:
ACK 4ead0adcb5 successfully ran local tests
tcharding:
ACK 4ead0adcb5
Tree-SHA512: 1e4411fdf019f3793e6366eda7e8b46246e3263bd7e41802877c20cc5e8ea451a79d89b4c59204ea9c1eb590054975de52a791a9d17a1d180a5cfac316efa959