add a tree-like system to run sets of benchmarks

A system a bit similar to `OUnit`, with the ability to select only a subset of the benchmarks to run them. As each benchmark can take a significant time, it is convenient to be able to run only the ones that are relevant to some function, say, if we are trying to improve this function's performance.

There is also a small patch to `_oasis` because it wouldn't compile with a recent version of `oasis` otherwise. 
Not sure to understand the interest of this function.  Isn't calling `map_int [f1, i1; f2, i2;...]` equivalent to `concat [f1 i1; f2 i2;...]` — the execution of the benchmarking code being delayed anyway because this is the point in wrapping it in a `Tree.t`?