For a flexible attribute query, replace the `col_name` property with
a function call that extracts that attribute from the `field_attrs`
field introduced in the earlier commit.
Additionally, for boolean, numeric and date queries CAST the value to
NUMERIC SQLite affinity to ensure that our queries like 'flex:1..5' and
'flex:true' continue working fine.
This removes the concept of 'slow query', since every query for any
field now has an SQL clause.
External Python packages interfacing beets may want to use an in-memory
beets library instance for testing beets-related code.
The `TestHelper` class is very helpful for this purpose.
Previously `TestHelper` was located in the `test/` directory.
Now it is part of `beets` itself (`beets.test.helper.TestHelper`) and
can be easily imported.
