utils: Move base16 procedures to (guix base16).

utils: Move base16 procedures to (guix base16).

* guix/utils.scm (bytevector->base16-string, base16-string->bytevector):
Move to...
* guix/base16.scm: ... here.  New file.
* tests/utils.scm ("bytevector->base16-string->bytevector"): Move to...
* tests/base16.scm: ... here.  New file.
* Makefile.am (MODULES): Add guix/base16.scm.
(SCM_TESTS): Add tests/base16.scm.
* build-aux/download.scm, guix/derivations.scm,
guix/docker.scm, guix/import/snix.scm, guix/pk-crypto.scm,
guix/scripts/authenticate.scm, guix/scripts/download.scm,
guix/scripts/hash.scm, guix/store.scm, tests/hash.scm,
tests/pk-crypto.scm: Adjust imports accordingly.