The primary addition is the `builtin_hashFile` function which attempts
value-path coercion and on success reads the file to a nix_string.
Similar to the `builtins_hashString` implementation, the hash is then
derived using the `hash_nix_string` function in the introduced hash
module which has extracted hashing functionality initially specified in
cl/11005.
Change-Id: I5e3ef0317223af99488ebb79efc2fb49b4cbc1b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11007
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>