Various Performance Improvements

A few days ago, Armin discovered Gnuplot. He wrote a script that turns the results of the nightly benchmark runs into plots (lower is always better, all the numbers of the microbenchmarks are "times slower than CPython"). The corresponding microbenchmarks can be found in the repository. Staring at the plots revealed a strange performance regression around the revision 45000. After some investigation Armin found that an mostly unrelated change had disabled our method cache, which caused the regression. This was fixed. In addition, Armin did a few other small tweaks in the interpreter main loop, making sure that small bytecodes are inlined into the main loop. This gave another few percent of performance increase. Together with the GC improvements two weeks ago this leads to the fastest non-JIT PyPy ever. Unfortunately "fastest" is not really very fast yet in absolute terms, with realistic apps being around 3-4 times slower than CPython. Especially calls (in all its variants) are quite slow, which is something we should look into.