Notably, pass around the location of the xcvb-program we create in the cache.

This allows the examples to be created without having to install
an XCVB. The primary implementation is in bootstrap.lisp and
additional code in driver.lisp.
+ The created XCVB is made with one of the supported lisp
implementations even though the enclosing lisp invocation might
not be.
+ Refactored lisp-invocation into its own package.
+ The cache-description file contains notes about how the cache
should function. The cache is both the multi-user cache as in
distcc, ccache, etc, but also the user specific workspace for
where to dump fasl files and whatnot.