Switchmap operator

[matiboy]

Minor docs amendments

Reviving the feature of switch_map proposed in #634 ; however I followed the RxJS implementation whereby:

1. The projector receives the map's index (use map_index instead of map)
2. There is no default projector. I don't feel that either identity or of is relevant, but would like to hear your thoughts on that since that was part of the conversation on Add switch_map operator and equivalent starred and indexed #634 @giff-h

[coveralls]

Coverage: 93.433% (+0.04%) from 93.396% when pulling 2a7d721 on feat/switch_map into a10cc1c on master.

[dbrattli]

1. Should we call it switch_map_indexed? This would align with flap_map vs flat_map_indexed
2. If you make the mapper optional, then you will automatically get the identity behaviour because of map_idexed where the mapper is optional.

[dbrattli]

Great work. Thanks for adding!

[MainRo]

@dbrattli can we merge this PR?