pp_return(): tail call pp_leaveeval()

pp_return(): tail call pp_leaveeval()

When 'return'ing from an eval STRING, rather than handling it
ourselves, fall through to pp_leaveeval(). pp_return() is now only
responsible for popping any extra contexts and junk from the stack.

This helps avoid two different blocks of code doing roughly the same
thing.

The functional changes caused by this commit signify the divergence over
time between pp_leaveeval and the try-ish parts of pp_return. After this
commit, a return will:

    * now do an PERL_ASYNC_CHECK();
    * be smarter about not unnecessarily creating mortal copies
      of returned args;
    * restore PL_curpm *before* the LEAVE() rather than after.

The first two are probably good things; I'm not sure about the latter; it
may well be a regression, but nothing tests for it. At least it's
consistent now.