Added support for direct generics

Added support for direct generics

Direct generics are a rare phenomenon, the only notable example in GLib
is GArray. Direct generics are handled using macros/sizeof and passed
around by pointers (not in pointers), so they need not fit in a pointer.
Thus, values like double, int64 or even funny structure types can be
stored in GArray (unlike GPtrArray or GHashTable).

This commit implements a complete support of direct generics. Also, the
GLib VAPI is adjusted for direct generics.

Direct generics are triggered by a new parameter to CCode. They are only
supposed to be used in VAPI files, using them in normal source files is
undefined. If your type is something else than a GArray, reconsider the matter
twice before enabling direct generics.