renaming rfc4122 functions

[Mikopet]

Hey!

My second contribution is a small rename of certain obsolete function names. (namely, contains rfc4122)

The changes are cosmetic, does not carry real value...
... but it has some documental changes and additions regarding deprecation messages.

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Majority of the functions were private, but there was also a few public ones. This latter ones got proxy functions in place to not break BC, with the proper deprecation warning.

During my testing it works as expected... also, I took the liberty and put the next version number into the deprecation messages, indicating there will be a patch version published.

When we will have version 2.0, we could clean up deprecated features.

Preferably should be merged after #753

[Mikopet]

as per CONTRIBUTING.md states, I hereby pinging one of you. ( @kinggoesgaming )

Also suggesting checking on my other PR #753

[KodrAus]

Thanks for the PR @Mikopet. I can see the motivation for doing this, especially since the old RFC is now obsolete in favour of 9562. I think the name rfc on its own isn't specific enough to tell what it should be so we could make it rfc9562 instead. The old RFC got almost 20 years of mileage out of it so I don't think we're at a big risk of needing to do this a second time anytime soon.

Having said that, I'm not fully convinced we should do this. I personally find these kinds of pure-renaming refactorings mostly just a nuisance as a user. I think it's something to keep up our sleeves though if we find a better API in the future to push users to.

[Mikopet]

I think the name rfc on its own isn't specific enough to tell what it should be so we could make it rfc9562 instead. The old RFC got almost 20 years of mileage out of it so I don't think we're at a big risk of needing to do this a second time anytime soon.

I agree, sort of. name rfc is kinda meaningless unless somebody is familiar with it. However, the name rfc9562 is kinda the same, just feels like code smell to me. Perhaps a name like gregorian would make sense. (as the RFC in fact call it that way multiple times)

I'm not fully convinced we should do this. [...] just a nuisance as a user. I think it's something to keep up our sleeves though if we find a better API in the future to push users to.

No disagree here, this PR indeed is some bikeshedding. More like an attempt to push the crate in the direction for some cleanup efforts.. as it is a little bit lorn... yet, it is very much on the important side of all crates. (top 100)

I do have some new ideas for a new API, would mean a major release. I will share it on the relevant PR soon (when the PoC is working as expected)

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so, back to this PR. What do you think about a rename to from_gregorian_timestamp()/gregorian_to_unix()/etc?
If you are up for it, I adjust the PR... if not, just close it. no hard feelings :)