1/ Replacing tink.Registry().Blah() with tink.Blah().

2/ Updating the APIs and code structure, following latest Java APIs. The Golang implementation was developed following Java 1.0.0, but many things have changed since then.

Next:
* Adding missing APIs such as KeysetReader, KeysetWriter.
* Removing tink.CleartextKeysetHandle().
* Adding RegistryConfig.
* Adding missing primitives such as envelope encryption, hybrid encryption, etc.
PiperOrigin-RevId: 196266656
GitOrigin-RevId: f4fe243c4501017922eaed1b464da70d3936c7ac