DataItem improvements, clippy clean, deps & edition bump

PR is made of three commits for eventual cherry picking:

1. data_item.rs:
   • Made DataItem fields public and added Default.
   • Added DataItem::is_ok().
   • Exposed DataItemBuilder and added Default (related to 2.).
2. Fixed several clippy burps.
3. Bumped dependencies & bumped edition to Rust 2021.
4. Updated the ATR doc sample to show use of the init struct pattern pattern with DataItem.
5. Merged with @menkaru's branches. I.e. added HMA, WMA & QQE.

Any plans to get this upstream @greyblake ? :)

@virtualritz Sorry, I appreciate your effort, but I won't be able to merge this. It's too much of everything in one PR. Regarding DataItem I've answered https://github.com/greyblake/ta-rs/issues/52, i won't accept the init struct pattern, since it does not favor correctness of use.

Regarding the indicators, I would appreciate if you open separate PRs (one PR per indicator), so I can thoroughly review it.

I hope for you understanding. Thanks.