Thread API cleanup

This patch cleans up the thread API and comes with the following
noteworthy changes:

- Introduced Cpu_session::Weight type that replaces a formerly used
  plain integer value to prevent the accidental mix-up of
  arguments.
- The enum definition of Cpu_session::DEFAULT_WEIGHT moved to
  Cpu_session::Weight::DEFAULT_WEIGHT
- New Thread constructor that takes a 'Env &' as first argument.
  The original constructors are now marked as deprecated. For the
  common use case where the default 'Weight' and 'Affinity' are
  used, a shortcut is provided. In the long term, those two
  constructors should be the only ones to remain.
- The former 'Thread<>' class template has been renamed to
  'Thread_deprecated'.
- The former 'Thread_base' class is now called 'Thread'.
- The new 'name()' accessor returns the thread's name as 'Name'
  object as centrally defined via 'Cpu_session::Name'. It is meant to
  replace the old-fashioned 'name' method that takes a buffer and size
  as arguments.
- Adaptation of the thread test to the new API

Issue #1954

repos/base/src/lib/ldso/main.cc

@@ -567,9 +567,9 @@ void Component::construct(Genode::Env &env)
 	try {
 		if (Genode::config()->xml_node().attribute("ld_verbose").has_value("yes")) {
 			PINF("  %lx .. %lx: stack area",
-			     Genode::Thread_base::stack_area_virtual_base(),
-			     Genode::Thread_base::stack_area_virtual_base() +
-			     Genode::Thread_base::stack_area_virtual_size() - 1);
+			     Genode::Thread::stack_area_virtual_base(),
+			     Genode::Thread::stack_area_virtual_base() +
+			     Genode::Thread::stack_area_virtual_size() - 1);
 			dump_link_map(Elf_object::obj_list()->head());
 		}
 	} catch (...) {  }