treesum: REFACTOR: Major refactoring of computing digests: - Digests that are created by reading symlink targets are created as if the result of readllink() is the content of a regulare file: no special tagging is done now. - All digest values have their canonical algorithm name prepended. - Normalize the order of computation

# Pure-Python-Build
[build-system]
requires = ["setuptools>=43.0", "wheel>=0.33"]
build-backend = "setuptools.build_meta"