add Porting/bench.pl

This tool runs code snippets found in t/perf/benchmarks (or similar) under
cachegrind, in order to calculate how many instruction reads, data writes,
branches, cache misses, etc. that one execution of the snippet uses. It
will run them against two or more perl executables and show how much each
test has gotten better or worse.

It is modelled on the perlbench tool, but since it measures instruction
reads etc., rather than timings, it is much more precise and reproducible.
It is also considerably faster, and is capable or running tests in
parallel. Rather than displaying a single relative percentage per
test/perl combination, it displays values for 13 different measurements,
such as instruction reads, conditional branch misses etc.

This commit also changes the format of t/perf/benchmarks slightly; it
becomes an AoH rather than a HoH (to allow checking for duplicate keys),
and the test names themselves become a :: hierarchy.