Investigate proto.Clone performance impact

[kruskall]

Local benchmarks show proto.Clone taking a small amount of CPU time (~8% total time). It's probably not enough to cause a regression but it's not great that something we introduced is taking a noticeable CPU time as it would decrease the impact of other performance improvements.

Clone is using reflection under the hood, we should try to minize its usage.

[axw]

https://github.com/planetscale/vtprotobuf

clone: generates the following helper methods

func (p *YourProto) CloneVT() *YourProto: this function behaves similarly to calling proto.Clone(p) on the message, except the cloning is performed by unrolled codegen without using reflection. If the receiver p is nil a typed nil is returned.

[kruskall]

Closed by #87