add t/perf/benchmarks, t/perf/benchmarks.t

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.