This adds EnsureHashAlgorithm constraint which checks whether the
given algorith name is contained in hashlib's algorithms_guaranteed
set. It derives from EnsureChoice without further customization.

Using `algorithms_guaranteed` covers algorithms that have named
constructors in hashlib, i.e. accessible through getattr(hashlib,
name) - that's the current usage in MultiHash. Future expansion could
see us move to algorithms_available - these will be recognized when
passed to (slower) hashlib.new(), but have no named constructors.

For details, see: https://docs.python.org/3.11/library/hashlib.html

The EnsureHash algorithm now calls its super.__init__ correctly.

We use EnsureHashAlgorithm() | EnsureListOf(...) in ls_file_colelction
now. This correctly handles incoming values, which are always a list
in CLI (argparse behaviour with --append) and can be str or list in
Python API. At least when they are OK.

When input is not OK, two things are suboptimal:
- an incorrect hash (str) proceeds to EnsureListOf, which tries to go
  letter by letter, failing on first
- a hash (list) with an incorrect value would first fail on
  EnsureHashAlgorithm(list), proceed to EnsureListOf, and fail on
  EnsureHashAlgorithm(str)
As a conequence, in both cases the error message would say "does not
match any of 2 alternatives" and print EnsureChoice message ("is not
one of ...") twice.

as suggested in
datalad#346 (comment)