Bags

[Ludovic Courtès]

Hello,

I’ve pushed a new branch, ‘wip-bags’, which changes the way
build systems work.  Basically, it introduces an intermediate
representation between packages and derivations, called “bags.”

The main difference between a bag and a package is that a bag lists all
the inputs of the build process, including the “implicit inputs” added
by the build system.

This simplifies the code and should unlock nifty features like ‘guix
environment’ and “grafts” (a trick for faster security updates; more on
that later.)

Comments welcome!

Ludo’.