Adding benchmarks to shoes4

This Pull requests adds a `benchmark` directory to the shoes4 main directory.

These benchmarks are meant to compare our performance to that of the good old red shoes.

What does it show so far?

* just creating paras seems to faster than in shoes3
* URLs are significantly slower (30 times visiting two simple pages: 3.263000 vs. 0.009616)
* The URL slowness most likely comes from the fact that multiple calls to clear (which URL uses) are ridiculosely slow, so for a rather complex page we have (shoes4 vs. shoes3):
  * 1. call to clear: 0.094000 vs.  0.001722
  * 2. call to clear: 50.539000 vs. 0.001137
  * 3. call to clear: 49.210000 vs. 0.000457

This PR is not about the performance problems, I'll open an issue for that, it is about getting this directory and these benchmarks in.

Cheers,
Tobi

PS: Like super happy I've finally found what causes these immense perf problems in a repeatable way