profiles: Support the creation of profiles with version 3 manifests.

profiles: Support the creation of profiles with version 3 manifests.

* guix/profiles.scm (%manifest-format-version): New variable.
(manifest->gexp): Add optional 'format-version' parameter.
[optional, entry->gexp]: Honor it.
(profile-derivation): Add #:format-version parameter and honor it.
(<profile>)[format-version]: New field.
(profile-compiler): Honor it.
* guix/build/profiles.scm (manifest-sexp->inputs+search-paths): Support
both versions 3 and 4.  Remove unused 'properties' variable.
* tests/profiles.scm ("profile-derivation format version 3"): New test.