* common.scm: add the constant `%time-relative-forever`.
* container/metadata.scm: add `#:filename` and `#:narinfo` meta-types.
* fs.scm: add tools to create and manipulate `file-information` objects:
`file-information-add!`, `file-information-iterate`,
`file->file-information` and `directory->file-information`,
remove `make-file-information`.
* tests/fs.scm: test those functions a little.
* identity.scm: `open-identity-service` now throws an exception on failure.
* binding-utils.scm: just add `destructuring-bind`.
* common.scm: `time-rel` now throws an exception instead of returning a
meaningless negative result; add `setup-log`.
* container/metadata.scm: `metadata-set!` now throws an exception on error.
* tests/container-metadata.scm: add tests for `metadata-copy`,
`metadata-clear`, `metadata-equal?`
and `add-publication-date!`
Rémi Birot-Delrue