#5 replace the gnunet scheduler with a guile-fibers implementation

Done, modulo bugs and some missing functions. Let's keep (rehash fibers) in-tree for some longer to work out the kinks, then extract to a new repo at https://notabug.org/mdevos/fibers-gnunet.

All functions are implemented, but interrupting twice yields a segfault

(#< base-ops: #(#< wrap-fn: #f try-fn: # block-fn: #> #< wrap-fn: malloc(): smallbin double linked list corrupted

It is possible a bug in the test suite is at cause of never making a connection to the rehash service. (The test case is a single thunk executed under do-gnunet-stuff and it sleep. TODO: fiberised rehash API)