Add Sequel.deep_qualify, for easily doing a deep qualification

There are a few places in Sequel where deep qualification is done
internally, but it wasn't previously accessible without creating
a Qualifier object manually.  This adds DSL support for it, which
should make it easier to use.
Fix bad spec description
Add pg_array_associations plugin, for associations based on PostgreSQL arrays containing foreign keys

Note that until PostgreSQL adds support for enforcing referential
integrity for foreign keys in array columns, it's probably a bad
idea to use this in production.
Ensure Dataset#unbind returned variable hash uses symbol keys

If a SQL::Identifier or SQL::QualifiedIdentifier contained a string,
it previously used a string key, but bound variable hashes should
always have symbol keys.  This caused breakage in the
prepared_statements_associations plugin when associations used
conditions with such values.