Refine API and protocol

This PR contains the following API improvements:

• Rename copy_free to zero_copy. Fixes #1 (closed).
• Add argument for number of compression options. Fixes #2 (closed).
• Require compression options in protocol metadata

It is still not possible to change default compression options during serialization. Where possible without additional work (lz4, deflate), non-default compression options are supported during deserialization.

While using a list of integers for the compression options in the protocol metadata looks reasonable, a future PR might add a more human friendly API to the library.