Similar to create() in LibJS, wrap() et al. are on a low enough level to
warrant passing a Realm directly instead of relying on the current realm
from the VM, as a wrapper may need to be allocated while no JS is being
executed.
If a C++ object already has a JS wrapper, we don't need to go through
the expensive type checks to figure out which kind of wrapper to create.
Instead, just return the wrapper we already have!
This gives a noticeable increase in smoothness on Acid3, where ~10% of
CPU time was previously spent doing RTTI type checks in wrap(). With
these changes, it's down to ~1%.
