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. 
>  these functions return `samples Lazy.t`

But then, you will loose the ability to run a given test more that once... (not sure that you really want to do that still, could you give a scenario?).  I'll have to think about it a bit more — and rest first! ;-)