Cep21 Circuit Versions Save

A major version v4 release of circuit library. See the changes in this PR: https://github.com/cep21/circuit/pull/121

For an up to date guide on upgrading v3 -> v4, see https://github.com/cep21/circuit/blob/master/UPGRADE_GUIDE.md.

A short list of changes are:

• Drop support for Gopkg.toml
• Remove the "/v3" root directory
• Move statsd implementation to another library
• Add ctx to the stats interfaces
• Move benchmarks to their own repo
• Use Go's built in atomic package
• Add upgrade guide

Reduce number of imports by giving benchmarking exclusive go.mod.

I copy the license file to v3 directory so pkg.go.dev can see it correctly.

Slowly figuring out all the go module corner cases for the right way to do this.

Supports a golang module file