Unconditionally define Enumerator::Lazy methods in our impl.

Our impl was borrowed from Marc-Andre Cournoyer's "backports" impl
which attempted to detect whether it was running on a Ruby that
already had several lazy Enumerator methods defined already. The
logic to do so triggered a thread-based Enumerator (using
Generator) to be created for at least one method detection and
possibly others. This caused some of our trace function tests to
intermittently see one or more Generator thread's death throes,
inserting bogus entries into test results.

This fix removes the detection logic since we know we'll always
need to define the backport version of these methods. No Generator
gets started during boot now.

It is possible this will also slightly improve startup, since that
logic was using slow-path threaded Enumerator logic.