Missing typings

[tdecker91]

There's still a few missing typings in index.d.ts. The following methods are contained in the docs here https://selfrefactor.github.io/rambda/#/ but don't have typings.

groupBy
indexBy
indexOf
lastIndexOf

Changes I'd like to introduce #82

[selfrefactor]

Thanks for the PR. I accepted it and the changes will apply to the next version.