Collage RFC (#62)

* Collage RFC

* - Add some FAQ entries. Make the partition_for_x preamble issue more explict.

* - Clarify pass ordering issue.

* - Strawman for moving CollageFuseOps early.

* - Switch from 'fusion' to 'partition' focus, matched prototype again.

* - Err, three advantages, not four.

* - minor clarrification

* - some more clarifications based on github comments

* - expand on PartitionRule

* - Manupa's comments

* - Remainder of Manupa's comments (oops)
- Add some pictures

Author: mbs-octoml