It should be possible to use older bootstrap and client code

[quicklisp]

When there is a potentially disruptive update to the bootstrap and client code, it should be possible to do the following:

• Enumerate which older bootstrap & client files are available
• Choose an older version during bootstrapping
• Choose a specific older version of the client after bootstrapping

Quicklisp already provides a system for choosing older sets of libraries. Making it possible to use older code in other respects will reduce the risk inherent in updating the bootstrap and client code, too.