[MERGE] refactor sub returns (pp_return etc)

This series of commits attempts to reduce duplication and partially unify
the code that handles subroutine exit, in places like pp_leavesub,
pp_return etc.  In particular, pp_return has been heavily modified so that
it is now only responsible for doing any extra work required above and
beyond that done by pp_leavesub et al (popping contexts and removing junk
left on the stack). It now tail calls pp_leavesub / pp_leaveeval/ etc to
do the remaining heavy lifting. This reduces the code size of pp_return to
about a quarter of what it was previously, and ensures that exactly the
same processes happen regardless of whether an explicit return is is done
or not.

It also fixes using return in list context in a MULTICALL, e.g.

    use List::Util qw(pairmap);
    @a = pairmap { for (1,2) { return (3,4)} } qw(a b);

formerly returned (1,2,3,4) and now returns (3,4).

It also simplifies S_sortcv() et al which are responsible for calling
a sort sub.

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