Log Message:
textproc/guile-json: update to 4.7.2

Changelog:

    record: also fix nested record for vectors

    bump version to 4.7.2

    record: don't call scm->record if value is *unspecified* in
    (define-json-type)

    Given the following example:

       (define-json-type <account>
         (id)
         (username)
         (omitted "omitted" <omitted>)
         (boolean))

       (define-json-type <omitted>
         (name))

    If `omitted` is *unspecified* there's no need to keep going through
    that field.

    This also rolls back part of commit 029212e.

    Fixes #78