In HNSW some nodes may become disconnected
from the rest of the graph. New `unreachable_nodes`
API allows retrieving the number of such entries.
On a real-world Wiki1M dataset around 0.1% nodes
are isolated.

Sometimes, knowing `disconnected_nodes()` number is not enough.
To know exactly which nodes are disconnected from the member
nodes of the current level, the upgraded API returns a `bitset_t`.

Moreover, multiple nodes may have links referencing each-other
and forming a connected component, but still be disconnected from
the rest of the hierarchical graph. To detect those - a
new `unreachable_nodes()` API was added.

This patch also refactors `bitset_gt` and `ring_gt`, extending
their functionality and fixing compilation errors.