Faster feature checks

Faster feature checks

Perform only a bit check instead of a much more expensive hash
lookup to test features.

For now I've just added a U32 to the cop structure to store the bits,
if we need more we could either add more bits directly, or make it a
pointer.

We don't have the immediate need for a pointer that warning do since
we don't dynamically add new features during compilation/runtime.

The changes to %^H are retained so that caller() can be used from perl
code to check the features enabled at a given caller's scope.