Avoid vivifying stuff when looking up barewords

Till now, when a bareword was looked up to see whether it was a sub-
routine, an rv2cv op was created (to allow PL_check hooks to override
the process), which was then asked for its GV.

Afterwards, the GV was downgraded back to nothing if possible.

So a lot of the time a GV was autovivified and then discarded.  This
has been the case since f74617600 (5.12).

If we know there is a good chance that the rv2cv op is about to be
deleted, we can avoid that by passing a flag to the new op.

Also f74617600 actually changed the behaviour by vivifying stashes
that used not be vivified:

sub foo { print shift, "\n" }
SUPER::foo bar if 0;
foo SUPER;

Output in 5.10:

    SUPER

Output as of this commit:

    SUPER

Output in 5.12 to 5.21.3:

    Can't locate object method "foo" via package "SUPER" at - line 3.
Update B::Concise for OPpMAY_RETURN_CONSTANT
Increase $B::Concise::VERSION to 0.993
Stop turning off bit 1 in rv2cv private flags

newUNOP turns on that bit by default, to indicate that the op
has one kid.

rv2cv was using that bit for something else, so three code paths had
to turn it off.

Two commits ago, rv2cv stopped using that bit, so turning it off
is no longer necessary.  It was because the flag using that bit,
OPpMAY_RETURN_CONSTANT, needed to be passed to newUNOP, that it had
to change.
Fix up Peek.t after priv flag twiddling