feat(tvix/eval): Implement builtins.fromJSON

Using `serde_json` for parsing JSON here, plus an `impl FromJSON for
Value`. The latter is primarily to stay "dependency light" for now -
likely going with an actual serde `Deserialize` impl in the future is
going to be way better as it allows saving significantly on intermediary
allocations.

Change-Id: I152a0448ff7c87cf7ebaac927c38912b99de1c18
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6920
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/default.nix

@@ -106,11 +106,15 @@ lib.fix (self: depot.third_party.naersk.buildPackage (lib.fix (naerskArgs: {
 
           base="$(dirname "$i")/$(basename "$i" ".nix")"
 
-          if [[ "$(basename "$i")" == "eval-okay-search-path.nix" ]]; then
-            # TODO(sterni): fix this test
-            echo "SKIPPED: $i"
-            continue
-          fi
+          case "$(basename $i)" in
+            eval-okay-search-path.nix) ;&
+            eval-okay-fromjson.nix)
+              # TODO(sterni,grfn): fix these tests
+              echo "SKIPPED: $i"
+              continue
+              ;;
+            *) ;;
+          esac
 
           if test -e $base.exp; then
               flags=