Enhancements to cli list and describe commands

Specifically:
- Add `\d` to describe any sequence, table or view (logical pair of `\l`)
- Add `\dq` for describing a sequence (logical pair of `\lq`)
- Add indexes (table and group) and references (FK and GFK) to `\d` output
- Add Group Path column, from information_schema.grouping_constraints.path, to `\lt` 

I took the editorial liberty of of *not* printing the constraint name of the GFK, as it is long and mostly useless and foresee adjusting that when we store proper constraint names.

Also use the opportunity to rewrite/cleanup how the backslash queries are used and stored. Basically, just stuff the query parts into an object so the client code can pass it around instead of redundant switching.

Note: fdbe183 means this new client can't execute `\dt` against SQL Layers < 1.9.3. A version check wouldn't be horrible if that seems useful.