Add methods to operate on several items at once

- Added `findIndices` and `removeSeveral` methods.
- `findIndices` was modified from `findIndex`.
- `removeSeveral` was modified from `removeAt`.

I don't know where you want tests, so there aren't any, but I can confirm that they work for my current project.

Let me know if I need to/should change anything!

(Thanks again for writing Cortex.)