User-friendly output of thread IDs and MHP

[sim642]

Since #331 and #518 the output of races has become much harder to read.

1. The thread IDs with history can be long and even when they're simple and unique, there's unnecessary internal notation, e.g. ([main], {}).
2. The MHP component also contains the thread ID (which in the default configuration is the same duplicated) and two sets of such verbose thread IDs (created and must joined).

We should think of some neater way to output these such that the race output isn't such incomprehensible wall of text.