CodeCov support

[TheAngryByrd]

Description

It would be great to have CodeCov support added. Once Expecto gets support we should add it. haf/expecto#219. Here's an example repo doing it in C#/F# https://github.com/codecov/example-csharp/ https://github.com/codecov/example-fsharp

[TheAngryByrd]

Here's a dotnet tool with some of this already. Need to investigate: https://github.com/lucaslorentz/minicover

[baronfel]

And now another: https://github.com/tonerdo/coverlet

[TheAngryByrd]

I did some investigations haf/expecto#219 (comment) and haf/expecto#219 (comment)

I haven't tried https://github.com/tonerdo/coverlet because it's tied to dotnet test but the experience looks nice. Maybe we can apply it to altcover

[TheAngryByrd]

Essentially done via #85

Using altcover and reportgenerator. If people want to hook up to CodeCov it shouldn't be much work from there.