add testing framework for boolean context

add testing framework for boolean context

Some ops (currently just padhv and rv2hv) are optimised when found to be
in boolean context - by setting a private flag on the op indicating
definite or maybe boolean context. At run time, the op can just return
true / false rather than a real value, which may be cheaper.

This commit adds a bunch of tests in nested loops to optree.t to check
that the right private flags are set for the various permutations of

    if (%h || $x) { ...}

etc.

It's written in such a way that its easy to add new ops to it.

At the moment many permutations are actually commented out, as these
(fairly comprehensive) tests show up a number of deficiencies in the
current implementation. These should be fixed in the next commit.