support labels indexing and hint generator

The idea is to further narrow search domain for node by support label indexing by default.

```python
class LabelIndexer:
pass

def motif_to_indexer_ast(motif):
for cname, jsondata in motif.nodes(data=True):
ast = None
# handle non label here

# handle label here
labels = list(jsondata.get("__labels__", set()))
ast = IndexerAnd(
ast,
IndexerOr(labels)
)
return ast

def match_iters(self):
entity_domains = IndexRunner.find(ast, attr_indexer, label_indexer)
```