Revert "Test: 31"

This reverts commit 9a42616a652c80fba74c62e116b81c5a40146d23.
Revert "Let's try current TPP list first."

This reverts commit 294cbc6eaed753bb1564ae84c3685da13737cb0a.
Test: 30. Let's back to this case and think about another transformation.
TPP: array -> container. Few things to note here:
1. I refactor the if condition (number > 20) to (number <= 20) so that return WORDS[number] becomes the first execution path
2. I refactor the array to a Map which contains the same data as array. Only thing changed here is the array index now becomes the map key.
3. Then, I transform the code to add another pair (30 -> "thirty") into map to pass the test case 30. At this step, I need to change the if condition to WORDS.containKey(number). I treat this change and the new key-value pair both as a necessary part of this array -> container transformation. With them, the code is "really" using a container now.
Move test case 30 to two digits should return one word
Test: 31
TPP: expression (actual a constant 20) -> function (closestOneWordNumber)
Meanwhile, WORDS, the container, is refactored to "NavigableMap" to support this transformation