Bump MSRV to 1.70

[marmeladema]

This project follows Tokio MSRV which is now at 1.70.
Additionally, this allows to use panic in constant to simplify some code.

[seanmonstar]

Hm, that comment in the README is likely wrong, this more closely follows hyper's policy: https://github.com/hyperium/hyper/blob/master/docs/MSRV.md

Which I'm not so sure I'd want to increase it for better panic messages, as Debian stable still has 1.63.

[marmeladema]

Thank you for your inputs.
Bumping to 1.63 would still be enough to get const panics.
I will update the proposal to that version but if that's still not appropriate, feel free to close this.