Provide information about packages required by TeX string.

The `check` function now returns `<foo>_required` fields to indicate
whether certain TeX packages are required to format the given string.

The change in commit 36a7aec80c35269bbc504c0b57196fe7c61fcf97 which
changed the return value of the `parse` method was reverted, since
the extra information was added to `check` instead of `parse`.