Push/pop Timer name prefixes

[vasdommes]

Currently one has to declare Timer name as "run.solve.some_function.other_function".

We can automatically store current prefix in Timers (and push/pop prefixes using RAII).
Then the code will look like

{
  Scoped_Timer root_timer(timers, "root") // "root"
  Append_Timer_Prefix root_prefix(timers, "root");
  {
    Append_Timer_Prefix prefix(timers, "nested"); 
    Scoped_Timer timer(timers, "step_1"); // "root.nested.step_1"
    Scoped_Timer timer(timers, "step_2"); // "root.nested.step_2"
  }
  Scoped_Timer timer(timers, "foo"); // "root.foo"
}

Inspired by
https://gitlab.com/bootstrapcollaboration/blocks_3d/-/commit/8717ba22dea47d7b13cff43ad02da83bff559b38#2ad9be1f8a0f77401abdb81cd016327598b6dd51
(but we can do it in a more elegant way)

[vasdommes]

We can even ask Scoped_Timer to append its name to current prefix.
Seems that we (almost?) always want a 'root' timer for a given scope.

NB: This works only if child timer is guaranteed to finish before the parent one. With Scoped_Timer, it will almost always be the case (unless we do some dirty tricks with pointers-to-timers). Simple Timer is more flexible, easier to break this nesting.