Add initial implementation of partial evaluation

These changes introduce a new evaluation mode in topdown that allows
callers to mark input, data, or variables as unknown. The result of
partial evaluation is a new set of queries that can be executed when the
inputs become known. When topdown partially evaluates a query, it saves
expressions that it cannot evaluate so that they can be returned to the
caller.

As part of these changes, the binding list has been updated so that
variables can be mangled when a plug operation is performed. This allows
variables to be correctly namespaced when they're inlined into parent
queries as part of partial evaluation.