0.6.0 release

This is major release featured in part 2 of ECS tutorial, check it out!

The changes include:

• ability to set system execution order;
• various improvements and new features to the entity printing facility;
• QoL helpers SPEC-ADJOIN, COPY-ENTITY and REPLACE-COMPONENT;
• slightly better syntax for :ARGUMENTS clause in DEFSYSTEM;
• a bunch small fixes and improvements;
• dependency on a new trivial-adjust-simple-array library.

Thanks to Daniil Kisel, Markus Klink and Alcor for bug reports and discussions.

See changelog for a full list of changes.