Improve ElementType ergonomics

[pnehrer]

Minor improvements to ElementType ergonomics:

• derive Copy, Clone to make it easier to pass around as a type vs. u32
• derive PartialEq, Eq to make instances comparable
• implement TryFrom with a custom error
• implement conversion Into u32
• implement Display

If you accept, I have a follow-up PR that would change some of the u32 return values to ElementType in Node and Tensor, but this would be a breaking change.

[abrown]

@pnehrer, thanks again for the contribution! I'm going to merge #99 first since that was sitting in the queue and that might cause you a bit of rebasing headache — hopefully not too much. And feel free to replace those u32s with ElementType since we haven't yet published a version that contains these new 2.0 changes.

[pnehrer]

@abrown I resolved the merge conflict and this is now ready to go

[pnehrer]

Thank you again!