This makes the 'Node' API public, and available for use directly for
plugins.

This replaces the helper method by adding a 'get_provenance' on the node
directly

- Adapt all call sites
- Delay getting provenance wherever possible without major refactor

Using 'Node.from_dict({})' can replace new_empty_node, and the rest
is not needed anymore.

- Adapt all call sites

- Also add a convenience method 'safe_del' catching the exception
when we don't care if the value was there or not.

- Adding 'get_sequence' on MappingNode to access sequences in a mapping
- Adding 'sequence_at' on SequenceNode to access sequences in a sequence
- Adding 'mapping_at' on SequenceNode to access mappings in a sequence

Using "*_at" in sequences allows us to quickly understand if we are
dealing with a sequence or a mapping.

- 'get_scalar()' allows retrieving a scalar node from a mapping.
- 'as_str()' casts a ScalarNode into a string (thus removing the node
  information).

Both together, those replace 'node_get(mapping, key, type=str)' but
also allow retrieving the 'Node' itself, which will allow in the future
lazier provenance computation.

This allows to get a mapping node from another 'MappingNode',
replacing 'node_get(my_mapping, key, type=dict)'

Also changes all places where 'node_get' was called like that by
the new API.

Instead of having methods in Context forward calls on to the Messenger,
have folks call the Messenger directly. Remove the forwarding methods in
Context.

If the junction source consistency is cached but the junction is not in
the source cache, also use Stream.fetch_subprojects() to import sources
into source cache instead of doing this implicitly in
Element._stage_sources_at(). This is in preparation for buildbox-casd,
which will disallow local CAS cache writes from the main process (gRPC).