Rename sort-by, add grouping, refactor+fix some join stuff

`sort-by` is now called `sort`. This is just because I think it makes more
sense and nothing more.

Grouping has been added with `group` and `having`. They behave as you'd expect
from SQL, except that joining with a table which has a `group` or `having`
clause will move that query in as a subquery. I wasn't sure how else to ensure
their composition, so this way maintains the semantics (which is the most
important part) at the potential cost of some efficiency in performing the
query. I'm not sure that there exists a more general solution. I would assume
not.

The join stuff has been fixed a bit (some valid renames were rejected as
invalid) and standardised a bit (exceptions are now more similar). A bunch of
things have been added to the join stuff as a result of the above grouping
things, too. A bunch of changes all around, basically.

The compiler has also had a few small changes made to it, and has been enhanced
to support the grouping stuff.

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