add t/perf/, t/perf/opcount.t

Add a new directory designed to hold performance / optimising tests
and infrastructure, and add the first test file, opcount.t, that
checks that a sub has the right numbers of particular op types
rename t/op/opt.t -> t/perf/optree.t

Now that we have a directory, t/perf/, for perfomance /optimsation
tests, move this test file there, and rename to something slightly
clearer.
t/perf/optree.t: expand blurb

explain (kind of) why this file is called optree.t
add t/perf/speed.t

This test file is similar to /re/speed.t, but to test general-purpose
optimisations.

The idea is to run snippets of code that are 100s or 1000s times slower
if a particular optimisation is broken. We are not so much interested
in the individual tests passing, as in the whole file failing with a
watchdog timeout (or just observing that it running more slowly)
add t/perf/benchmarks, t/perf/benchmarks.t

t/perf/benchmarks is a file intended to contain snippets of code
that can be usefully benchmarked or otherwise profiled.

The basic idea is that any time you add an optimisation that is intended
to make a particular construct faster, then you should add that construct
to this file.

Under the normal test suite, the test file benchmarks.t does a basic
compile and run of each of these snippets; not to test performance,
but just to ensure that the code doesn't have errors.

Over time, it is intended that various measurement and profiling tools
will be written that can run selected (or all) snippets in various
environments. These will not be run as part of a normal test suite run.
add note about t/perf/ to t/README
fix 't/TEST -benchmark'

This has never worked, as it would look for t/benchmark/*.t files in
the wrong place.
[MERGE] add t/perf/ hierarchy

Add some new files that help with testing and measuring
performance-related issues.