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superseded by the doc of swh.model.identifiers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

This commits adds support for parsing optional parts in persistent
identifiers. Optional parts are defined the following way:

	<swh_id>[;key1=val1][;key2=val2]

Related T1040

Related T335

Related T335
Related T933

Closes: T335

reincarnation of the old shw.model bin/ script, which is now gone

Summary:
Add support for snapshot identifiers

Close T566.
Related to D268.

Test Plan: Unit tests included

Reviewers: zack, #reviewers!

Maniphest Tasks: T566

Differential Revision: https://forge.softwareheritage.org/D277

Close T709

This allows loaders to lazily load data: you can read from disk and transfer
only the contents that are really missing.

Summary:
This module is a reimplementation of swh.model.git, with the underlying goal of
replacing it and fixing T709 in the process.

Reviewers: #reviewers!

Differential Revision: https://forge.softwareheritage.org/D248

Currently just a stub (but with a hereby fixed anchor) to allow linking to it
from other part of the development documentation. Will be fillen later on with
an abstract description of the Software Heritage data model.