Monad code changes

[Ludovic Courtès]

Commit e87f059 moved monadic procedures “where they belong”–essentially
grouping them by abstraction layer rather than by programming style,
which meant everything in (guix monads).

This has then allowed the conversion of the code to download the package
source (the ‘method’ field of ‘origin’) to use monadic style, in commit
f220a83.

So the API is slowly but surely converging to a unified style.  The last
step will be to convert build systems to monads + gexps, hopefully in
the next ‘core-updates’ because that will require rebuilds.

Ludo’.