FCurve: optimize search from an RNA path + index.

By checking the index value first instead of a full fledge string
comparision in `BKE_fcurve_find`, we can make that code significatly
faster (from about 10% in a Heist production file to over 45% in a
heavily animated test file).

While this code was already very fast (a few microseconds per call
typically), it gets called a lot from the UI (several hundreds of time
per refresh), among other things.

NOTE: the `UNLIKELY` hint is responsible for 25% to 30% of the
speed improvement.

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