Remove unused track_length parameter

And revert to their original behavior (returning only digest as bytes)

This allows calls like:

- MultiHash.from_file(file_object).digest()
- MultiHash.from_path(b'foo').hexdigest()
- MultiHash.from_data(b'foo').bytehexdigest()

This is the first step to improve the hashutil module according to
"the" plan [1].

In this regards, the hashutil exposes the same function as before.
Internally though, they now uses a MultiHash instance.

Related D410

[1] D410#7952

Related T421

Related T421

Related T1152

... and add an option to explicitly not follow them, if needed.

Closes T1135

Closes T1134

with explicit option to hide it

Closes T1133

This commit changes the swh pids representation returned by the
parse_persistent_identifier function from a dict to a named tuple.
Also, the 'object_type' attribute of that named tuple now contains
a full name instead of an abbreviated one.

Closes T1112
Closes T1125

Close T1104
Close D346

Related T1104

Related T1104

As the client of this api might not be aware of what's the right id
key for a given object.

Related T1104

Related T1104

Related T1104

Related T1104

Summary:
We used to depend on the 'blake2s256' and 'blake2b512' names to be available in
hashlib.algorithms_available. It turns out that that's specific to OpenSSL 1.1.

We now try, in order:
 - blake2s256/blake2b512 as shipped by libssl1.1 (Python 3.5+ on Debian stretch
 and up)
 - blake2s/blake2b as built into Python 3.6+
 - pyblake2 if all else fails

While we're here, let's also avoid doing not-so-subtle hacks with hashlib
builtins.

Thanks to Alexios Zavras for the report.

Test Plan:
New unit tests added to check for behavior in all cases. Manually running the
tests on Python 3.4 + pyblake2, Python 3.5 and Python 3.6 as shipped by Debian
exercises all three cases.

Reviewers: zack, #reviewers!

Differential Revision: https://forge.softwareheritage.org/D347

Currently only content and directory object types are supported, but more can
be added in the future.

Closes T1039

superseded by the doc of swh.model.identifiers

text adapted from iPRES 2017 paper about Software Heritage, section "Data
Model"

Closes T787

Closes T1041