Implement default Random instance.

Previously, the default random number generator was implemented using static
storage.  This storage is common to all instances of mruby in a process, and
use of the default random number generator in one instance will perturb the
default random number generator in other instances.  It is also not thread
safe.

With this change, the default random number generator is defined as
Random::DEFAULT, as it is in CRuby.