Added support code for comfortable writing of upstream data update
expressions that do not require change of layout of the updated
expressions (although they make assumptions about single assignment per
line). Also added a default for choosing file to update (it is supposed
to be default.nix in the same directory) and a one-liner for typical
sourceforge redirects (and sourceforge mirror:// handling).
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.
