Remove slot and rename session to slot

Yes, this is confusingly similar to reverting the recent commits, but slots are
now playing the role of sessions.
WIP: new attribute/measure schema; broken
Switch kind name fields to enums to be smaller and eliminate Checks
WIP: more comments and audit tables for annotations
Improved SQLArgs and Audit interface; annotations WIP broken
More functions for mapping annotations to targets

Also wrote a generic Asset.get.  All untested.
Minor bug fixes on comments and ranges.

Haven't I learned the val-val dependency problem by now?