* guix/combinators.scm (memoize): Remove.
* guix/memoization.scm: New file.
* Makefile.am (MODULES): Add it.
* gnu/packages.scm, gnu/packages/bootstrap.scm,
guix/build-system/gnu.scm, guix/build-system/python.scm,
guix/derivations.scm, guix/gnu-maintenance.scm,
guix/import/cran.scm, guix/import/elpa.scm,
guix/modules.scm, guix/scripts/build.scm,
guix/scripts/graph.scm, guix/scripts/lint.scm,
guix/store.scm, guix/utils.scm: Adjust imports accordingly.