Raise precedence of "is". Fix #119.

test/is/is.wren

@@ -28,9 +28,6 @@ IO.print(new Fn { 1 } is Class) // expect: false
 IO.print("s" is Class) // expect: false
 IO.print(123 is Class) // expect: false
 
-// TODO: Non-class on RHS.
-// TODO: Precedence and associativity.
-
 // Ignore newline after "is".
 IO.print(123 is
   Num) // expect: true

test/precedence.wren

@@ -22,6 +22,10 @@ IO.print(false == 2 <= 1) // expect: true
 // >= has higher precedence than ==.
 IO.print(false == 1 >= 2) // expect: true
 
+// is has higher precedence than ==.
+IO.print(true == 10 is Num) // expect: true
+IO.print(10 is Num == false) // expect: false
+
 // Unary - has lower precedence than ..
 IO.print(-"abc".count) // expect: -3