mutable decl syntax change

examples/programs/control_flow/main.bro

@@ -23,7 +23,7 @@ noisy func() bool {
 }
 
 main func() i32 {
-	total i32 = 0
+	total i32 := 0
 
 	# comparisons drive if / else if / else
 	total = total + classify(-5)   # 1
@@ -45,9 +45,9 @@ main func() i32 {
 	}
 
 	# block scoping: inner bindings do not escape the block
-	x i32 = 1
+	x i32 := 1
 	if x == 1 {
-		inner_x i32 = 100
+		inner_x i32 := 100
 		if inner_x == 100 {
 			total = total + 5      # 40
 		}