Fix strict const_err

[HeroicKatora]

An array is indexed with constants that exceed its size. This is
inopportune even in a branch that never gets executed. This changes it
so that the indexing happens on a slice. This is a potential forward
compatibility hazard that isn't too hard to fix.

Also removes a bunch of annotations by avoiding primitive casts with
explicit type annotations in constants.

[HeroicKatora]

Waiting on some feedback from rust: rust-lang/rust#65005

[HeroicKatora]

So looks like const_err is a deny-by-default lint and lints are allowed to be arbitrarily broadened. Since the 'bug' in question exists since 0.19 and versions before only work because they are buggy 😆 . I'll probably backport to 0.21 like before, it's still about equal to 0.22 in terms of users, slowly declining.

The better fix probably involves adding allow(const_err) to the macro in question since I'm not sure if the allow changes may not even potentially allow evaluating self.0[..][4] as if a const expression, despite the slice type in the middel.

[HeroicKatora]

So, the difference between deny(lint) and an error is that the former still gets silenced when image is used as a non-path dependency. Phew. So no risk of downstream crates breaking (until it gets turned into a hard error which is likely not soon).

[HeroicKatora]

And I found a cute way to avoid the issue regardless. Since old versions as dependencies won't break there is no reason to backport except if someone complains because they use it by path.