[bug#46124] [PATCH] Idris 2

[raingloom]

Now that Idris 2 has had some proper tagged releases, I thought I'd
move it from my channel to Guix proper.
I've been using this package for quite a few months now and it pretty
much works. REPL works, building things works. Couldn't try IDE
functionality but I suppose it probably works if you can set it up in
Vim. That reminds me, I still haven't packaged the Vim mode, but I
don't use Vim so I don't really know its module system.

## Still TODO for the package:

search paths: There are no 3rd party Idris 2 modules yet, but it should
be done soon.

output prefix weirdness: Output is a bit odd, having an idris2-0.3.0
directory in its root, but since this is Guix, I suppose that's not
really a problem. Everything else is in its proper place, in bin, lib,
and share.

clang-toolchain support: Right now CC=gcc is hardcoded. Auto detection
would be preferable.

bootstrap: Right now it's bootstrapped from auto-generated Chez or
Racket. It **can** be built with Idris 1, but it uses such on obscene
amount of RAM that I refuse to consider building it that way ever
again. If someone wants to incorporate that step into Guix, they are
free to do so. Or we could generate it once and then freeze it and use
it as an input. Just leave my poor laptop out of it, it already
suffered enough. And probably leave the CI infrastructure out of it too.


## Bigger TODO:
Build system. Eventually it will need one.
Considering that it has multiple code generation backends, this is not
trivial, as each backend uses a different existing language, the
official ones (that I can remember right now) being:
* Chez
* Racket
* Gambit
* Chicken
* JavaScript/Node
* C

It might also make sense to compile Idris 2 itself with different
backends. It "officially" supports bootstrapping with Chez and Racket,
but others might be possible too.


Anyways, here's the patch, have fun with it, and/or suggest changes,
etc!