Update benchmark

Add v2

Go requires a separated folder for v2+ to keep the
compatibility for tools that are not version-aware.
https://go.dev/blog/v2-go-modules

Removed Abool.Toggle to mitigate a regression to Abool.SetToIf

Add IsNotSet()

Refine comments

Update benchmark to Go 1.14.3 on Linux

Assuming that Linux is a more popular platform than Darwin when running
this package

Golang is not Go