instruction: Fixup instruction data transmute

#### Problem

The `InstructionData` helper creator doesn't work for all possible
types, since the `@sizeOf` macro does not use packed size.

#### Summary of changes

Use `@bitSizeOf` instead, and add a test.

src/instruction.zig

@@ -72,12 +72,38 @@ pub const Instruction = extern struct {
 ///     .data = data.asBytes(),
 /// });
 pub fn InstructionData(comptime Discriminant: type, comptime Data: type) type {
+    comptime {
+        if (@bitSizeOf(Discriminant) % 8 != 0) {
+            @panic("Discriminant bit size is not divisible by 8");
+        }
+        if (@bitSizeOf(Data) % 8 != 0) {
+            @panic("Data bit size is not divisible by 8");
+        }
+    }
     return packed struct {
         discriminant: Discriminant,
         data: Data,
         const Self = @This();
         fn asBytes(self: *const Self) []const u8 {
-            return std.mem.asBytes(self)[0..(@sizeOf(Discriminant) + @sizeOf(Data))];
+            return std.mem.asBytes(self)[0..((@bitSizeOf(Discriminant) + @bitSizeOf(Data)) / 8)];
         }
     };
 }
+
+test "instruction: data transmute" {
+    const Discriminant = enum(u32) {
+        zero,
+        one,
+        two,
+        three,
+    };
+
+    const Data = packed struct {
+        a: u8,
+        b: u16,
+        c: u64,
+    };
+
+    const instruction = InstructionData(Discriminant, Data){ .discriminant = Discriminant.three, .data = Data{ .a = 1, .b = 2, .c = 3 } };
+    try std.testing.expectEqualSlices(u8, instruction.asBytes(), &[_]u8{ 3, 0, 0, 0, 1, 2, 0, 3, 0, 0, 0, 0, 0, 0, 0 });
+}