add `removeAt` function

(cherry picked from commit 6c7cc2ef389db1f4b2074e735bf318e3ce3d34cd)
fix `_.removeAt` with repeated indexes
use more descriptive variable name
use `else` instead of `continue`
Add `_.pullAt`. [closes #490]
Tweak docs for `_.pull`, `_.pullAt`, and `_.remove`. [ci skip]
Whitespace nit.