Have a path highlighter for the call tree

[Navidem]

It can be useful for the call tree report to have a button besides each function (along [function][call_site]) to display the reachability path from the fuzz entry to that function.
It also can colorize the covered/non-covered functions along the path.

It can show something like:
LLVMFuzzerTestOneInput (GREEN) --> foo (GREEN) --> bar (RED) --> target_func (RED).

[DavidKorczynski]

Good idea.

I think it may make sense to broaden the scope and add a dedicated page for a "trace" in the calltree. A page that provides information about a given individual trace, e.g. more detailed insights about blockers on the path, source code snippets for fast tracing (instead of having to navigate constantly between calltree and coverage report), which fuzzers reach the various nodes in the calltree, runtime density, etc.

[DavidKorczynski]

Something related is happening in #764

There the goal is to show which paths lead to a given sink function. At the moment it shows a simple textual trace for each possible path, but I think it would be nice to improve that too with color highlighting etc.