assert: added noreturn attribute to error_abort().
config: added platform and architecture detection definitions.
doc: reworked groups; corrected group inclusion around namespaces;¹ excluded impl.

1: Doxygen ¬_¬  Apparently you have to place @addtogroup within a namespace.
It probably doesn't handle bodies very well.
test/premake: corrected index base for test object.
threading: new module; added Thread and a POSIX implementation.
test/premake: always apply togo-strict for test projects.
threading: added thread::is_main(), return "main" from thread::name() when on main thread; expanded test/threading/thread.
threading: moved Thread into separate doc-group.