collate: fix Compare[String] funcs to use key comparisons

[gburt]

Change collator.Compare and collator.CompareString funcs to just do bytes.Compare
on the keys of the two strings.

I'll be honest, I don't know why Compare/compare() was implemented how it was versus
doing what I'm doing here. Looks like it was last touched in 2013 when @mpvl moved it to
this repo from somewhere else.

And add unit tests for the UCA Variable Weighting examples (https://www.unicode.org/reports/tr10/#Variable_Weighting_Examples). I had thought they
weren't implemented at all because 1) CompareString was broken and 2) I wasn't adding
the "-ks-level4" language tag option to actually enable Shift-Trimmed fully. Correcting those
two things and they all pass!

Also add a comment about how the Collator is not safe to use concurrently.

Fixes golang/go#68166

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