Create godbolt library

[seanbaxter]

In order to use godbolt I manually copied all the headers into single-header/std2.h. Godbolt doesn't support remote headers with dependencies. To do that, we need to define a library and upstream it:
https://github.com/compiler-explorer/compiler-explorer/blob/main/docs/AddingALibrary.md

I changed the CML to point to single-header. Let's make changes to single-header/std2.h until we have a CE library checked in. But it does seem less convenient than single-header because you need a versioning strategy.