indexOf should return a Maybe, not -1

[remexre]

silver:core:indexOf should probably return a Maybe to ensure that callers are handling the case where a substring isn't found correctly.

This could be a performance regression; ideally we'd have a (static) inliner and something that can do the case-of-case optimization... but that's of course not part of this ticket, just wontfix this if it measurably regresses performance.