Convert fallible From implementations to TryFrom

[xla]

Currently there are a handful of cases where the From trait is implemented and the implementation can panic. This is usually addressed with a conversion to a TryFrom implementation. See 71f0cd3 and https://rust-lang.github.io/rust-clippy/master/index.html#fallible_impl_from

[liamsi]

Some of these Froms aren't used anywhere. Whoever tackles this might also consider removing some of the impls.

[tarcieri]

Which ones? It’d be good to make sure the KMS isn’t using them before removing them.

You can always add a temporary [patch.crates-io] to the KMS’s Cargo.toml pointing it at a local path to double check that.

[thanethomson]

Is this still relevant? Looking through the codebase now, I don't see any instances where we're disabling the fallible_impl_from lint, and I think there's one From impl that calls unwrap() internally (converting a u64 to an i64) and there's a TODO above it to fix it.