Lazily import implementations

[dahlia]

This fixes ImportError that setup.py throws:

$ python setup.py 
Traceback (most recent call last):
  File "setup.py", line 6, in <module>
    import hanja
  File "hanja/__init__.py", line 8, in <module>
    from hanja.impl import is_hanja, is_valid_mode, split_hanja, translate
  File "hanja/impl.py", line 5, in <module>
    from hanja.table import hanja_table
  File "hanja/table.py", line 2, in <module>
    import yaml
ImportError: No module named yaml

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[dahlia]

@googlebot I signed it!

[coveralls]

Coverage increased (+0.7%) to 93.846% when pulling 5c1a3c2 on dahlia:make-setup.py-independent into e1ec45b on suminb:develop.

[dahlia]

A build job (pypy) failed because the cryptography package was failed to be installed. I believe the job fail is irrelevant to this pull request. Other build jobs all succeeded.