This is a slightly modified version of ladislav-zezula/StormLib@4ad0bff21da0163917e1552960e6a43679586b89

Cosmetic changes:
* Remove all unused files.
* Comment out unnecessary bits with `#ifndef FULL`.

Implementation changes:
* Redefine `bool` to `BOOL`.
* Use STORMAPI instead of WINAPI for calling convention (and define it as nothing).
* Use external Storm error handling (`SErrGetLastError()` etc.)
* Convert pathnames to Unix-style slashes.
* Bugfixes, with comment: // BUGFIX (devilutionX)
* Fix missing header includes on macOS (devilutionX)