[client]: improve batch request API (#910)

* better handling of batch requests

* add test for untagged enum

* remove annoying trait bounds

* cleanup

* more clear comments

* simplify batch request code

* bring back old API + a new one

* refactor batches to work with String IDs again

* refactor again: single batch response API

* fix tests + cleanup

* fix doc links

* address grumbles

* BatchRequestBuilder: add iterator API for the batch

* revert bench

* fix benches build

* address grumbles: ok and into_ok

* fix some nits

* fix nits