Move generic Subtag to subtags

[zbraniecki]

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First patch from the #1833 patchset.

It moves the Subtag to locid::subtags::Subtag - this is a generic subtag (2..=8, separate from private::Subtag which is 1..=8) which will be used to iterate over multi-part subtags like extensions::unicode::Value to retrieve specific items without relying on the underlying TinyStr storage.

It's a breaking change because we are removing extensions::other::Subtag. I assume this is ok as we're working on 2.0 so I don't need to add alias and deprecate it, but please confirm.

[robertbastian]

Breaking changes are not ok yet. I'm working on releasing 1.5, after which the breakage can begin.

[zbraniecki]

what's the ETA for 1.5?

[robertbastian]

The next few days