chore(motoko): migrate cert-var to mo:core 2.4.0 (#1340)

- Add mops.toml: moc 1.5.1, core 2.4.0, -W=M0236,M0237,M0223
- Update dfx.json packtool to "mops sources"
- Replace mo:base imports with mo:core equivalents
- Drop unused Debug import
- Use x.toNat8() context dot instead of Nat8.fromNat(Nat32.toNat(x)), dropping Nat8 import
- Fix M0194: blobOfNat32 was using captured `value` instead of parameter `n`

Author: marc0olo

motoko/cert-var/dfx.json

@@ -20,7 +20,7 @@
   },
   "defaults": {
     "build": {
-      "packtool": ""
+      "packtool": "mops sources"
     }
   },
   "version": 1

motoko/cert-var/mops.toml

@@ -0,0 +1,11 @@
+[toolchain]
+moc = "1.5.1"
+
+[dependencies]
+core = "2.4.0"
+
+[moc]
+# M0236: use context dot notation
+# M0237: redundant explicit implicit arguments
+# M0223: redundant type instantiation
+args = ["-W=M0236,M0237,M0223"]

motoko/cert-var/src/cert_var/main.mo

@@ -1,10 +1,8 @@
-/// Simple counter (see `Counter.mo`), but uses `mo:base/CertifiedData` to
+/// Simple counter (see `Counter.mo`), but uses `mo:core/CertifiedData` to
 /// implement the counter value as a certified variable.
-import CD "mo:base/CertifiedData";
-import Blob "mo:base/Blob";
-import Nat8 "mo:base/Nat8";
-import Nat32 "mo:base/Nat32";
-import Debug "mo:base/Debug";
+import CD "mo:core/CertifiedData";
+import Blob "mo:core/Blob";
+import Nat32 "mo:core/Nat32";
 
 persistent actor Variable {
 
@@ -14,14 +12,12 @@ persistent actor Variable {
   /// LE encoding matches Candid encoding of Nat32, for consistency and convenience.
   func blobOfNat32(n : Nat32) : Blob {
     let byteMask : Nat32 = 0xff;
-    func byte(x : Nat32) : Nat8 {
-      Nat8.fromNat(Nat32.toNat(x));
-    };
+    func byte(x : Nat32) : Nat8 = x.toNat8();
     Blob.fromArray([
-      byte(((byteMask << 0) & value) >> 0),
-      byte(((byteMask << 8) & value) >> 8),
-      byte(((byteMask << 16) & value) >> 16),
-      byte(((byteMask << 24) & value) >> 24),
+      byte(((byteMask << 0) & n) >> 0),
+      byte(((byteMask << 8) & n) >> 8),
+      byte(((byteMask << 16) & n) >> 16),
+      byte(((byteMask << 24) & n) >> 24),
     ]);
   };