Split PaginatedResults into 2 classes, one sync and one async

[rgeronimi]

Ths is follows #95 where we patchd PaginatedResults to be 100% async. Its resulting code is a bit convoluted, as it mixes sync and async APIs, whereas other library classes (e.g., SemanticScholarcleanly separate the sync and async versions).

PS : for the future I think the PaginatedResults class could be split in 2 sync vs async versions. This would make the API and code much clearer than it is today, exactly like the SemanticScholar class.

It might be worth opening an issue or PR to discuss this further.

Originally posted by @danielnsilva in #95 (comment)