Add datastructure and tools for building and printing stack traces / ref trees conveniently

[ogxd]

Context

Building stacks will be a very common thing to do in profiler, either stacks by occurrence, stacks by thread group, reference tree, ... It's worth making something generic and convenient to use.

Example usage

Let's say we want to print stacktraces:
We make a sequences field in out profiler

pub sequences: HashMap<Vec<FunctionID>, usize>

Vec<FunctionID> is the stack, usize is the number of times this stack was encountered. This is convenient to use and very performant while the profiler runs since there is no search to do, everything is O(1).

When we're done profiling, we build a tree using

let mut tree = TreeNode::build_from_sequences(sequences)

Then we can sort our tree let's say by descending inclusive calls (this is usually what we want) with

tree.sort_by(&|a, b| b.inclusive.cmp(&a.inclusive));

Then it should be a lot easier to print our stacks into HTML or whatever

[Bluezen]

After reviewing the TreeNode structure in this pull request and providing some feedback, I must say that overall it was very well done. Great work!