utils: Add `fold-port-matches' and `remove-store-references'.

* guix/build/utils.scm (fold-port-matches, remove-store-references): New
  procedures.

* tests/build-utils.scm ("fold-port-matches", "fold-port-matches,
  trickier", "fold-port-matches, with unmatched chars"): New tests.

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'.