replace numpy usage with custom types using buffer protocol

[maxbachmann]

The library should not directly use numpy for the following reasons:

• numpy wheels are not available for many platforms, which makes compiling for these platforms a very slow process
• the numpy dependency is very large, which makes it harder to use the library in some contexts

This can be solved by writing custom containers which follow the buffer protocol. Those would be easy to convert to numpy arrays when needed without requiring numpy as dependency. This should be done for all new APIs: #188. For the existing cdist API this change would need to wait until version 3.0.0, since it is a breaking change. Alternatively cdist could be deprecated, since everything that can be done with it can be done with the new Compare API as well.