stdlib should be extracted into its own repo

[redbaron]

with multiple jsonnet implementations available, shuldn't stdlib reside in a dedicated repo, where it is then pulled in by implementations?

[sbarzowski]

I think @sparkprime proposed this when I started contributing to go-jsonnet, if not earlier...

Related: google/go-jsonnet#205
Related: google/go-jsonnet#207

[sparkprime]

Realistically, we would move the C++ implementation out of this one into some google/cpp-jsonnet or whatever