Replacing SBCL upstream tracking expression with a new version in a new
format.
Minuses: gave up on defining everything in Nix language (now update
expression is a series of actions to do when downloading fresh release,
it is actually interpreted by shell), now Nix expression contains
meaningful whitespace (the area to regenerate is determined by the
line with a specific comment and the closing brace on the otherwise
empty line).
Plusses: only one extra file which could even be moved out-of-tree if
desired, clean semantics for traversing multiple links (it is not found
in either Debian uscan or Gentoo euscan), the main expression is in one
file and is less different from usual style.
