Upgrade to 2.6.0, add IDNA functions to library

[bbayles]

Closes #22

This PR upgrades ada to version 2.6.0.

ada-url/ada#457 added the ada_idna_to_unicode and ada_idna_to_ascii to the C API. This PR adds Python wrappers for them. Some notes:

• Python users are familiar with this package, which provides idna.decode and idna.encode. I've exposed the functions under those name here.
• That package accepts either str or bytes types; I've made the same choice here
• That package emits str for the "to unicode" operation and bytes for the "to ASCII" operation; I've made the same choices here
• The functions are also available under the names idna_to_ascii and idna_to_unicode

[anonrig]

Amazing work as always @bbayles, and welcome to the parents club 💯🥇

[bbayles]

Fixed the docs in the latest commit - here is the rendered result:

---