Hi Guix,

This patch series is my suggestion for allowing
multiple "substitution methods" or "substituters"
as I call them.  Currently, only a method for HTTP/S
is defined, though I hope it will be a good basis
for a common framework for substitutes over GNUnet
and IPFS.

Extending "guix-service-type" to allow configuration
of substitution method is left for later.

Any questions, remarks?

Greetings,
Maxime