`formatter_float.bench.cpp` currently benchmarks the floating point
formatting very extensively. This patch reduces the number of benchmarks
by removing some of the cases that are relatively meaningless.
The benchmark is also converted to the more recent style of benchmarks.
As a nice side-effect, this reduces the time it takes to compile the
benchmark by ~20x.
We may be able to drop more benchmarks, but I'm not an expert here and
am rather conservative here for that reason.
