parser: simplify finishAssignment, add StarEqual support

examples/hello.v

@@ -58,6 +58,8 @@ fn main(a int) int {
 	// those should yield the same ast
 	str.len = str.len + 1
 	str.len += 1
+	str.len -= 1
+	str.len *= 1
 }
 
 fn (v Typ) voidfunc() {}