enforce integer division via explicit builtins

examples/programs/compound_assignment/main.bro

@@ -1,5 +1,4 @@
-# Milestone 6: compound assignment (`+=`, `-=`, `*=`, `/=`) and the binary
-# arithmetic operators `-`, `*`, `/` with multiplicative precedence.
+# Compound assignment (`+=`, `-=`, `*=`, `/=`) and explicit integer division.
 
 check_float func() i32 {
 	x f64 = 10.0
@@ -15,7 +14,7 @@ check_float func() i32 {
 
 check_unsigned func() i32 {
 	n u32 = 100
-	n /= 7 # 14 (truncating integer division)
+	n = div_trunc(n, 7) # 14
 	n -= 4 # 10
 	if n == 10 {
 		return 1
@@ -28,7 +27,7 @@ main func() i32 {
 	total += 10 # 10
 	total -= 3  # 7
 	total *= 4  # 28
-	total /= 2  # 14
+	total = div_trunc(total, 2) # 14
 
 	# binary operators honour precedence: 14 + (2 * 3) - 4 == 16
 	total = total + 2 * 3 - 4