tailing return type specification caused infinite template instantiation

This reverts commit dcf2c79a.

Added support for testing aql files (tests/aql).
Removed alib2integrationtests/ in favour of tests/ directory.

Sorry for a messy commit.
Closes #198.

It really makes no sense to have it inside alib2std component. Moved
into separate tests/ folder.

Run install target of the build process.

We used 'contrib' name improperly [1,2].

[1] https://drewdevault.com/2020/06/06/Add-a-contrib-directory.html
[2] http://www.geschke-online.de/doc/c2646.html

Simplify versioning by CMake. Uses git version in the format
"<major>.<minor>.<patch>.r<commits-since-version-tag>.g<hash>(-dirty)?".

Adapts Doxygen generators to changes in previous commit.

Remove CMakeLists.txt generator scripts from build phase. Replaced with
CMake's GLOB that can re-run every time the build is invoked. We can
also drop the Python dependency now.

Removed header installs which will be redesigned in future commits.