Make the ScopedAllocCounter public and document it

[gbin]

As Copper tries to be 0 allocation end to end during the execution time we developed a simple Allocation tracer to monitor that within Copper (and to eventually inform the tasks developers if they do allocate).

But it would be useful externally to, for example for benchmarks, because it was not well exposed for a quick benchmark I had to hack it from the GLOBAL here #143

Let fix that and make it a first class member of the Copper libraries.