Constify RzBuffer API

[Rot127]

Is your feature request related to a problem? Please describe.

The RzBuffer API doesn't pass RzBuffer pointers as const. Even if if it shouldn't (e.g. for rz_buf_size).

This is a problem. Because it prevents users of the API to also pass the pointers without const or do casts before using the API.

Both is not nice.

Describe the solution you'd like

Refactor the API to use const where possible.

Describe alternatives you've considered

None. I can't come up with a good reason to not do it.

Additional context

None