Add service-tree navigation methods to close #8.

The getAll...() methods would take a union type instead of being overloaded,
except ReSpec loses a level of HTML escaping inside unions, which breaks
sequences.
Update the explainer for symbolic names.

Also updated the definition of BluetoothScanFilter to allow symbolic names.
Merge pull request #13 from WebBluetoothCG/service-by-uuid

Add service-tree navigation methods to close #8.