utils: Move combinators to (guix combinators).
* guix/utils.scm (compile-time-value, memoize, fold2)
(fold-tree, fold-tree-leaves): Move to...
* guix/combinators: ... here. New file.
* tests/utils.scm ("fold2, 1 list", "fold2, 2 lists")
(fold-tree tests): Move to...
* tests/combinators.scm: ... here. New file.
* Makefile.am (MODULES, SCM_TESTS): Add them.
* gnu/packages.scm, gnu/packages/bootstrap.scm,
gnu/services/herd.scm, guix/build-system/gnu.scm,
guix/build-system/python.scm, guix/derivations.scm,
guix/gnu-maintenance.scm, guix/import/elpa.scm,
guix/scripts/archive.scm, guix/scripts/build.scm,
guix/scripts/graph.scm, guix/scripts/lint.scm,
guix/scripts/size.scm, guix/scripts/substitute.scm,
guix/serialization.scm, guix/store.scm, guix/ui.scm: Adjust imports
accordingly.
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