grokkability improvements for partitions

Summary:

• Added some mypy types, including making Partitions generic.
• schedule_partition_range now returns a list of partitions instead of a function that returns a list of partitions.
• PartitionParams -> Partitions. I don't 100% love Partitions as a replacement, but IMO it gets the meaning across better. PartitionParams makes me expect a dumb struct of parameters to some fancy function.

Test Plan: bk

Reviewers: dgibson, rexledesma

Reviewed By: dgibson, rexledesma

Differential Revision: https://dagster.phacility.com/D8612