[Feature Request] - allow customized panic handler in downstream crates.

[updogliu]

I am writing a PWASM smart contract which need to use a customized panic handler, which is impossible for now since pwasm-std has already defined one.

Can we hide mod pwasm_std::panic behind a separate feature panic, or move it to a separate crate, so that the users of pwasm-std can define their own panic handler when necessary?