Add support for tracing-flame

[ctron]

This PR is based on #48. Only the last commit belongs to this PR.

This adds support for tracing-flame. (See #47)

This PR works, but might need some work on the documentation. I am happy to follow up with it if you think the approach is good in general.

Here is how this works:

• Enable the tracing-flame feature on test-log (this by itself doesn't to much other than adding the dependency).
• Setting the TEST_LOG_FLAMES env-var to a path when running tests, will write traced out as {module}/{name}.folded.
• Those can be processed with inferno as described by tracing-flames

The result of this looks something like: