Rename panic_implementation attribute to panic_handler

[nasherm]

The panic_implementation attribute has been deprecated for RFC 2070 and renamed to panic_handler

[Ben-PH]

Forking now. I ran into this and didn't think to put the issue up. I'm getting a lot of value out of this, will feel good to contribute.

[Ben-PH]

found the pull-request

deprecating panic_implementation

[phil-opp]

Fixed in #467. Thanks for reporting @TuringDisciple!