Refactor and adapt output format of Timer

This patch adds 3 changes:
- Rely on spdlog::stopwatch instead of the 'raw' standard library clock
- Change the output format a touch to improve readability
- Rename of variables to express intent better

As of version 1.8.0 of spdlog a stopwatch is provided. I thought it
would be nice to justs rely on that one instead of rolling our own (even
if it's not that hard).

The other change is the output format. Instead of printing in the format
'[Timing] [debug] Execution of <caller>: <fn> took <time>ms', I changed
it to '[<caller>] [debug] <fn> took <time>s'. So instead of having a
separate Timing logger, the logger for the class is used. IMO, this
cleans up the log a touch, as my eyes don't have to search in different
columns, but it's minor. Also I use the default output format of the
spdlog::stopwatch, which is in decimal and seconds.

Also I used the change to rename the variables, as it was just a little
off.
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