This patch adds v3.5.0 of the community fork of the Lean Prover.  The
reason for adding a new (gnu packages lean) module is having a dedicated
place for all Lean-related packages, as I will be working on packaging
various Lean libraries and projects next.

This patch is the first of hopefully many to come, motivated by the
recent proposal for The Formal Methods in GNU Guix Working Group
submitted to guix-devel earlier this month [0].

[0]: https://lists.gnu.org/r/guix-devel/2019-12/msg00177.html