Move pbt-gen under the tools folder

[greg-szabo]

pbt-gen seems a standalone tool. I think it should be under the tools folder. Can we move it?

What's the definition of "done" for this issue? A: pbt-gen works and is moved under the "tools" folder. or B: A decision with a reason is documented to keep it where it is.

[thanethomson]

Maybe @shonfeder can comment on this in more detail, but I think it's in the root of the project because it's a support library used during testing (i.e. it's compiled into other crates), as opposed to a separate utility like the other tools.

[shonfeder]

Yeah, it's a library, not a standalone tool and it doesn't strike me as a "tool" in an obvious sense. Still, if it can be imported as a library from within the tools directory and there's some benefit to putting it there, I don't have an objection. But I also don't know what the problem with the current placement is or what would be gained by moving it.

In short, I think I have no grounds for an informed opinion on this issue, including on the motive for it :)