For details on composer autoload, see:

https://getcomposer.org/doc/04-schema.md#autoload

This helps move from a custom maintained autoload function to a standard
one built by composer. Composer's autoload function is widely used by
the larger PHP community and is well tested. To maintain backwards
compatibility, Autoload.php is still included, but emits a warning.

This helps moves the project towards adopting the community standard
PSR-4 autoloading:

https://www.php-fig.org/psr/psr-4/

This is first step towards moving the package to a namespace and the
composer PSR-4 autoloader. The "classmap" autoloader is no longer used
for tests.

As the tests are not part of the public API, this is backwards
compatible.

Uses the PhpCas namespace.

Refs #188