glob: Support square brackets in patterns.

* guix/glob.scm (wildcard-indices): Remove.
(parse-bracket): New procedure.
(compile-glob-pattern): Rewrite.  Support square brackets for sets and
ranges.
(glob-match?): Support sets and ranges.
* tests/glob.scm (test-compile-glob-pattern)
(test-glob-match): New macros.
Use them to rewrite the existing tests, and add new tests.