mutable decl syntax change

examples/programs/errors/main.bro

@@ -125,7 +125,7 @@ inline_detail func(value i32) i32 ! union(enum) {
 }
 
 main func() i32 {
-	acc i32 = 0
+	acc i32 := 0
 
 	a :: maybe(0) catch 7
 	b :: maybe(4) catch 99
@@ -166,11 +166,11 @@ main func() i32 {
 
 	acc = acc + a + b + c + d + e + f + g + h + i + j + k + l + m + n + o + p
 	acc = acc + pick(.left)
-	r Right = .right
+	r Right := .right
 	acc = acc + pick(r)
-	box BoxA = .a{8}
+	box BoxA := .a{8}
 	acc = acc + payload(box)
-	empty BoxB = .b
+	empty BoxB := .b
 	acc = acc + payload(empty)
 	acc = acc + payload(.a{9})