packages: Introduce <content-hash> and use it in <origin>.

* guix/packages.scm (<content-hash>): New record type.
(define-content-hash-constructor, build-content-hash)
(content-hash): New macros.
(print-content-hash): New procedure.
(<origin>): Rename constructor to '%origin'.
[sha256]: Remove field.
[hash]: New field.  Adjust users.
(origin-compatibility-helper, origin): New macros.
(origin-sha256): New deprecated procedure.
(origin->derivation): Adjust accordingly.
* tests/packages.scm ("package-source-derivation, origin, sha512"): New
test.
* guix/tests.scm: Hide (gcrypt hash) 'sha256' for proper syntax
matching.
* tests/challenge.scm: Add #:prefix for (gcrypt hash) and adjust users.
* tests/derivations.scm: Likewise.
* tests/store.scm: Likewise.
* tests/graph.scm ("bag DAG, including origins"): Provide 'sha256' field
with the right length.
* gnu/packages/aspell.scm (aspell-dictionary)
(aspell-dict-ca, aspell-dict-it): Use 'hash' and 'content-hash' for
proper syntax matching.
* gnu/packages/bash.scm (bash-patch): Rename 'sha256' to 'sha256-bv'.
* gnu/packages/bootstrap.scm (bootstrap-executable): Rename 'sha256' to 'bv'.
* gnu/packages/readline.scm (readline-patch): Likewise.
* gnu/packages/virtualization.scm (qemu-patch): Rename 'sha256' to
'sha256-bv'.
* guix/import/utils.scm: Hide (gcrypt hash) 'sha256'.

tests/graph.scm

@@ -162,7 +162,11 @@ edges."
   (let-values (((backend nodes+edges) (make-recording-backend)))
     (let* ((m (lambda* (uri hash-type hash name #:key system)
                 (text-file "foo-1.2.3.tar.gz" "This is a fake!")))
-           (o (origin (method m) (uri "the-uri") (sha256 #vu8(0 1 2))))
+           (o (origin
+                (method m) (uri "the-uri")
+                (sha256
+                 (base32
+                  "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"))))
            (p (dummy-package "p" (source o))))
       (run-with-store %store
         (export-graph (list p) 'port