Banish boolkeys

Banish boolkeys

Since 6ea72b3a1, rv2hv and padhv have had the ability to return boo-
leans in scalar context, instead of bucket stats, if flagged the right
way.  sub { %hash || ... } is optimised to take advantage of this.  If
the || is in unknown context at compile time, the %hash is flagged as
being maybe a true boolean.  When flagged that way, it returns a bool-
ean if block_gimme() returns G_VOID.

If rv2hv and padhv can already do this, then we don’t need the
boolkeys op any more.  We can just flag the rv2hv to return a boolean.
In all the cases where boolkeys was used, we know at compile time that
it is true boolean context, so we add a new flag for that.