settings: introduce structure helpers
settings: introduce structure helpers

This adds a few helper functions to deal with the Configuration's structure:
* has_value: Does the node contain a 'value' entry?
* has_default: Does the node contain a 'default' entry?
* is_leaf: Is the node considered a 'leaf' in the configuration? A leaf is a
node that either contains a value or a default entry.

simbuerg/benchbuild,simbuerg/benchbuild