Add inline qualifier in oop.hpp and fix inlining in gc files

Several recent changes cleaned up includes of oop.inline.hpp in real .hpp header files.

Unfortunately, the 'inline' qualifier is added to the function implementations in oop.inline.hpp instead of to the declarations in oop.hpp. Therefore, the compiler can not detect failing inlines properly.

This cleans up some remaining issues after the recent oop.inline.hpp fixes in the gc coding. Also, this change adds a few 'inline' keywords in oop.hpp. A later change to the 'rt' files will complete this and clean up all qualifiers in the oop.*.hpp files.