Add builder-side utilities for phases, stream editing, & co.

* guix/build/utils.scm (with-directory-excursion): New macro.
  (alist-cons-before, alist-cons-after, alist-replace): New procedures.
  (substitute): New procedure.

* tests/build-utils.scm: New file.

* Makefile.am (TESTS): Add `tests/build-utils.scm'.