This new interface is supposed to capture the core functionality of
DLTI: querying for values at keys. As such this new interface unifies
the ability to query DLTI attributes in a single method: query(). All
existing DLTI interfaces exposing their own query methods now 1) now
extend this new interface and 2) provide a default implementation for
`query()`.
As DLTIQueryInterface::query() returns an attribute, it naturally
enables recursive queries on nested DLTI attrs. A utility function,
`dlti::query()`, implements the logic for nested lookups.
A new `#dlti.map` attribute is introduced to capture the most generic
form of a finite DLTI-mapping. One of the benefits is that it allows for
more easily encoding hierachical information that is suitably queryable,
i.e. by means of nested attributes.
In line with the above, `transform.dlti.query` is modified so as to take
an arbitrary number of keys and to perform a nested lookup using the
above utility function.
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