* examples/search-ns.scm: a basic tool to search namespaces.
* examples/identity.scm: a basic tool to list egos.
* examples/identity-bis.scm: idem, but using `start-identity-lookup`.
* fs/uri.scm: `wrap-uri` throws an `invalid-arg` exception when given a
null pointer.
* tests/uri.scm: c.f. ↑
* configuration.scm: add `configuration-value-set?`.
* identity.scm: add `ecdsa-public-key->string`.