set up benchmark CI

[johnnychen94]

we don't really want to run the time-consuming CI for every PR/commits, so I set it to check if there's a "run benchmarks" label:

• if there's, run the benchmark and post the benchmark results
• otherwise, the benchmark CI becomes a trivial one that does nothing other than installing julia. -- This is unavoiable because GitHub action doesn't support workflow on labels.

let's see how this works out

[johnnychen94]

Great, it errors because master branch doesn't have benchmark/benchmarks.jl file, so I'll merge it now and test it against #27