Switch from `BatEnum` to `Seq`

[sim642]

In #975 some code was switched from eagerly-constructed lists to the standard Seq.t.
However, there's still many places where BatEnum/Enum is used, which offers similar lazy-sequence functionality.

We could try switching to the more standard Seq.t in those Enum instances, as long as there's no performance regression from doing so.

After #1443 and #1784, only a handful of instances remain:

• BatFile.lines_of and BatIO.lines_of. We'll have to find a sensible way of doing that.
• BatCache. We only use manual_cache which is just a thin wrapper around Hashtbl. We could easily just have a version in Goblint_std.
• BatEnum.uniq. There is no Seq.uniq or equivalent. We probably could just avoid that anyway by deduplicating smarter using sets.