Review unused public API

[armiol]

As tool-base is a set of modules, used by various Spine tooling, it exposes a big list of public types and methods.

Currently, some of them are detected as "unused". This is probably because they were used in tests of base repository — before they got extracted into this repo.

To avoid warnings — and to prevent someone accidentally killing the "unused" API elements — I have marked all of them as

@SuppressWarnings("unused") /* Part of the public API. */

And while this is true for most of the API endpoints, it may be false for some of them — and the latter really need to be killed.

In scope of this issue, the public API should be revised and "stripped-to-fit".