yfast trie Successor doesn't work well with big gaps between numbers

[xiankaing]

this test will fail

func TestTrieSuccessorBigGaps(t *testing.T) {
        yfast := New(uint64(0))

        e3 := newMockEntry(13 << 32)
        yfast.Insert(e3)

        successor := yfast.Successor(0)
        assert.Equal(t, e3, successor)

        e1 := newMockEntry(3 << 32)
        e2 := newMockEntry(7 << 32)

        yfast.Insert(e1, e2)

        successor = yfast.Successor(0)
        assert.Equal(t, e1, successor)

        successor = yfast.Successor(1)
        assert.Equal(t, e1, successor)

        successor = yfast.Successor(3<<32 + 1)
        assert.Equal(t, e2, successor)

        successor = yfast.Successor(8 << 32)
        assert.Equal(t, e3, successor)

        successor = yfast.Successor(14 << 32)
        assert.Nil(t, successor)

        successor = yfast.Successor(100 << 32)
        assert.Nil(t, successor)
}