Load UnicodeUtils partially to speedup gem load time

By requiring `unicode_utils/casefold` instead of `unicode_utils`, we gain around 350ms in gem load time without any loss in functionality.

Before:
```
>> puts Benchmark.measure { require 'carmen' }
  0.210000   0.140000   0.350000 (  0.352950)
```
After:
```
>> puts Benchmark.measure { require 'carmen' }
  0.020000   0.000000   0.020000 (  0.022928)
```

This also solves #141.