Previously, the sessions contained only explicitly exported variables
and functions.
This was fragile and in part buggy due to lacking exports.
Interactive features like user-defined aliases and functions are still
working as before.
This avoids an extra delay and the unexpected creation of secrets when
run in another dir.
Needed for the 'fetch-release' script introduced in a later commit.
Instead of forking this repo, it is now recommended that users simply import the
nix-bitcoin module. This commit adds an example directory that contains the
network/ examples and a shell.nix for deployment with nixops.
