For text files it is possible to do it like so: `builtins.hashString "sha256" (builtins.readFile /tmp/a)` but that doesn't work for binary files. With builtins.hashFile any kind of file can be conveniently hashed.
Fixes #679. Note: on x86_64, SHA-512 is considerably faster than SHA-256 (198 MB/s versus 131 MB/s).
adding primop function calculating hash of a string Signed-off-by: Marc Weber <marco-oweber@gmx.de>