Fixes for compilation-error-regexp-alist

- Makes matching of filenames of the form (foo.clj:XX) in stacktraces
  more accurate (avoiding some spurious highlighting in compilation
  buffers).

- Uses a more idiomatic entry for `compilation-error-alist`, namely a
  symbol which points to `compilation-error-regex-alist-alist`.

- Moves the code adding entries to the former variables to the
  top-level, instead of calling it over and over everytime a repl is
  started.