Group or collapse samples by leaf C++ namespace

Alongside the rough categories we have now, it would be helpful to all Gecko developers to group samples by the namespace of the leaf function, when it is C++. This would automatically distribute blame among categories of code meaningful to the programmer, like "js::" and "js::jit::" and "js::gc::". Better yet, it would help developers working in areas of the code base that we've never even heard of, who just happen to be using namespaces in their C++ code.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-237)