Optimize std.findSubstr and std.flattenArrays

[JoshRosen]

This PR implements small performance optimizations for two built-in functions:

• std.findSubstr:
  • Skip array allocation in case of no matches. This is a common scenario because findSubstr is often used to implement "string contains" checks.
  • Use ArrayBuilder instead of ArrayBuffer: in this context we only need to append and don't need to update, remove, or inspect the elements that we've written, and for those requirements ArrayBuilder is cheaper to instantiate and cheaper to invoke.
  • Store Val.Num into the array builder rather than storing indices then mapping the result into an output array: this saves an array allocation and iteration.
• std.flattenArrays:
  • Apply a similar ArrayBuffer -> ArrayBuilder replacement.