wip-build-systems-gexp

[Ludovic Courtès]

The new ‘wip-build-systems-gexp’ branch is where I intend to rewrite
build systems using gexps and ‘gexp->derivation’ instead of
‘build-expression->derivation’.  This would be the final part of the
move to gexps and the monadic API, which should improve consistency and
flexibility and hopefully reduce code duplication.

Currently there are still test failures and performance regressions, but
I hope to be able to fix those quickly, so this can be part of 0.8.2.

Comments welcome.

Ludo’.