[bug#41949] Adding the Gerbil scheme language, build system and two packages

[Edouard Klein]

Dear Guix,

The patches I'm about to send add the Gerbil scheme programming language
(https://cons.io), which could be described as Gambit, but with
batteries included.

The authors are friendly to Nix (they edited the source to make it
Nix-compatible) so I expect the addition to Guix to be welcomed there.

I've added the gerbil packages, which will allow one to start hacking in
Gerbil in no time, as well as the gerbil build system, as deployment
of software coded in gerbil is not easy. In theory static binaries are
possible, in practice it was easier to add gerbil to guix and use guix
pack ;)

Two packages are provided, to seed the addition of further packages to
the ecosystem, and to check that the build system works.

I'd be more than happy to apply the changes that will probably be
necessary (it's my first time working so deep in guix).

Cheers,

Edouard.