Add SourceLink for source debugging

[tmilnthorp]

Adding SourceLink per #532

[tmilnthorp]

I'm not 100% sure how this works so dunno how to test it :)

[angularsen]

Looks good to me, let's try it!

[angularsen]

@tmilnthorp I tried it and can't get it to work in Visual Studio 15.8.7, also running sourcelink test on it reveals some hashes don't match with the github URLs. Will need to look more into it.

Another thing I just realized; I don't think we can use SourceLink after we removed all the generated code - since that source code no longer exists on github. 😞 Maybe we should just stick to symbols nuget packages, which we already have.

[tmilnthorp]

Either that or we can add them back and use the following to clean up the GitHub diffs: https://robots.thoughtbot.com/github-diff-supression

This would be nice too so you don't have to rerun generate every time you check out, switch branches, etc.

[angularsen]

Let's continue discussion in #439.

[angularsen]

This was first reverted, then added back again in #548.