SI-8988 Escaping character in StringLike.split(c) is slow

Escaping a char when calling split is slow.  We end up compiling a
Pattern on each call to simply match a character literal.

Instead, we just use a loop with indexOf to construct our resulting
Array.

It's longer code, but according to my tests, this is about 11 times faster than what we had.
There is `assertSameElements`. I'm not sure about randString here. I'd use scalacheck if that's necessary. But actually it's a performance bug, not correctness, so in a way, nicer to have a fix+test that addresses just that.

https://github.com/scala/scala/blob/2.11.x/test/junit/scala/tools/testing/AssertUtil.scala#L34