When we started expanding and narrowing type sets, it became impossible
to conclusively know during pattern application whether a type was
known. We now figure that out at the end: if a variable has only a
single known type, we don't need to extract its type tag.
Part 1, core: use Rc for String and Keyword.
Part 2, query: use Rc for Variable.
Part 3, sql: use Rc for args in SQLiteQueryBuilder.
Part 4, query-algebrizer: use Rc.
Part 5, db: use Rc.
Part 6, query-parser: use Rc.
Part 7, query-projector: use Rc.
Part 8, query-translator: use Rc.
Part 9, top level: use Rc.
Part 10: intern Ident and IdentOrKeyword.
mod.rs defines the module and ConjoiningClauses itself, complete with
methods to record facts and ask it questions.
pattern.rs, predicate.rs, resolve.rs, and or.rs include particular
functionality around accumulating certain kinds of patterns.
Only `or.rs` includes significant new code; the rest is just split.
