Prevent exposing internal APIs as public

Currently this crate exposes every internal implementation detail as public API, making it almost impossible to do proper releases -- any internal change is seen as a public API change - thus breaking semver guarantees, forcing brotli crate to be published with a major version change. This fragments the community because cargo is unable to simply pick the later version of the crate for all usages in the same project.

To solve this, I would like to go through all of the APIs, and mark ALL of them as internal (crate-level), and then re-enable just the ones we explicitly want to publish. I am not sure if there is a tool for this, so I might have to go to the biggest users of the crate and see what APIs they use.