Implement the required endpoints for src# about doc-gen4

[hargoniX]

The general find# and src# system also needs to be reworked, this is because we generate a directory for every declaration but some declarations contain characters like ? which Windows doesn't allow in file/dir names :/. So in order to get Windows support we need to implement the find# and src# functionality fully in JS.

[gebner]

It's not just Windows, also on Linux there are restrictions on file names (depending on the file system, on ext4 a filename may not be longer than 255 characters).

[hargoniX]

I think at the point where a Lean declaration gets longer than 255 characters together with its namespaces the user probably deserves doc-gen to fail :P

[gebner]

I've tried to run doc-gen4 on mathport and ran into that issue with some autogenerated declaration. The new instance names (instA...) are also humongous, I wouldn't be surprised if they end up being longer than 255 characters in some cases.

[hargoniX]

Hmmm...well with the new approach of @xubaiw in #38 we should get rid of this subdirectory stuff in favour of just one endpoint with some JS running anyways so hopefully that won't be an issue anymore.