JSON for Hash now has to be Base16

Fix #14532.

As discussed on the call today:

1. We'll stick with `format = "base16"` and `hash = "<hash>"`, not do
   `base16 = "<hash>"`, in order to be forward compatible with
   supporting more versioning formats.

   The motivation we discussed for someday *possibly* doing this is
   making it easier to write very slap-dash lang2nix tools that create
   (not consume) derivations with dynamic derivations.

2. We will remove support for non-base16 (and make that the default, not
   base64) in `Hash`, so this is strictly forward contingency, *not*
   yet something we support. (And also not something we have concrete
   plans to start supporting.)

src/libutil-tests/data/hash/blake3-base16.json

@@ -0,0 +1,5 @@
+{
+  "algorithm": "blake3",
+  "format": "base16",
+  "hash": "9e70ee1449965fb62d049040a1ed06ec377430da6ec13173e7c4fffcd28be980"
+}

src/libutil-tests/data/hash/blake3-base64.json

@@ -1,5 +0,0 @@
-{
-  "algorithm": "blake3",
-  "format": "base64",
-  "hash": "nnDuFEmWX7YtBJBAoe0G7Dd0MNpuwTFz58T//NKL6YA="
-}

src/libutil-tests/data/hash/sha256-base16.json

@@ -1,5 +1,5 @@
 {
-    "algorithm": "sha256",
-    "format": "base16",
-    "hash": "f0e4c2f76c58916ec258f246851bea091d14d4247a2fc3e18694461b1816e13b"
+  "algorithm": "sha256",
+  "format": "base16",
+  "hash": "f0e4c2f76c58916ec258f246851bea091d14d4247a2fc3e18694461b1816e13b"
 }

src/libutil-tests/data/hash/simple.json

@@ -1,5 +1,5 @@
 {
   "algorithm": "sha256",
-  "format": "base64",
-  "hash": "8OTC92xYkW7CWPJGhRvqCR0U1CR6L8PhhpRGGxgW4Ts="
+  "format": "base16",
+  "hash": "f0e4c2f76c58916ec258f246851bea091d14d4247a2fc3e18694461b1816e13b"
 }