convert guava interface to libgap

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turns out to be straightforward, and much cleaner

CC:  @wdjoyner

Component: **coding theory**

Author: **Dima Pasechnik**

Branch: **[`f4b8c13`](https://github.com/sagemath/sagetrac-mirror/commit/f4b8c13c2f7a80fbc7bfcd2e8b75ad9e49e97418)**

Reviewer: **Frédéric Chapoton**

_Issue created by migration from https://trac.sagemath.org/ticket/28820_

