Remove some Identity conversions that could panic

[jsdt]

Description of Changes

This removes Identity::{from_slice,from_be_slice}, which previously had the potential to panic. Since callers can just use try_into, I don't think we need this in the API.

API and ABI breaking changes

This removes two functions from the Identity lib. If we find that these are widely used, we could add back a safe version. Making this safe will always be a breaking change though, since we need to change the return type to a Result.

Expected complexity level and risk

1

Testing

Not much added testing, since this is a pretty mechanical change.

[kazimuth]

This is a good idea. I was just retouching these and didn't like that unwrap. However, it is an API break. My concern was that from_slice might already be being used by clients so I wasn't sure about changing it. from_be_slice is definitely fine to remove since it was just added.

We could also change them to return Option<Self> instead.

[gefjon]

Thanks for cleaning this up. One minor terminology request: in the future, please don't use "unsafe" to describe Rust code unless it is unsafe, i.e. liable to invoke UB. Your PR title made me somewhat worried about this, when the actual changes are trivial and easy to approve.

[gefjon]

Spurious test failure should be fixed by #1979 . Please rebase onto master or merge master into this branch, at your preference, to confirm.