SI-8950  SeqView and StreamView allow indexing out of a slice

Added `idx >= 0` tests for `SeqViewLike#Sliced` `apply` and `mutable.IndexedSeqViewLike#Sliced` `unapply`.  Now you get `IndexOutOfBoundsException`s as you should.

No tests; this was found by collections-laws and will be tested by them.  (Exact variants in bug report were tested in the REPL.)