Update zeroize and MSRV accordingly

[nazar-pc]

Turns out similar to #386, but updates other relevant things.

I need this because it causes issues deep in the dependencies, my chain looks like this:

my-crate -> substrate -> libp2p -> snow -> curve25519-dalek

So this crate deep in dependency tree causes too many issues. I think bumping it in pre-release will not cause any issues for anyone in the ecosystem.

Closes #386, fixes #387

[fabioberger]

@tarcieri any reason this hasn't been merged yet?

[tarcieri]

I’m not a maintainer of this crate

[kimpers]

@isislovecruft would love to see this merged as this issue is blocking us from updating our dependencies

[fabioberger]

@isislovecruft is there a reason why this isn't getting merged?

[rozbb]

Resolved in release/4.0 branch