Added `WITH_REAL` versions to `RB_RANDOM_INTERFACE` macros. Also
these macros including "without real" versions no longer contain
the terminator (semicolon and comma).
* random.c: separate abstract rb_random_t and rb_random_mt_t for
Mersenne Twister implementation.
* include/ruby/random.h: the interface for extensions of Random
class.
* DLL imported symbol reference is not constant on Windows.
* check if properly initialized.
