(This bug is fixed upstream.)

We do require a recent toolchain [except for the compiler which is
stuck at 4.07.1 due to ppx_import]

32-bit compilation is using switch jscoq+32bit. 64-bit compilation
is using jscoq+64bit.
Coq is cloned into _vendor+xxx directories by Coq version and word
size, and so are add-ons.

We port the build system to Dune. This setup will integrate nicely
with Coq 8.10, but until this arrives it also provides a few
advantages:

- better `.merlin` files, automatically generated,
- clean build tree: all the artifacts go under _build,
- modular organization of the development: we introduce a separate
  library for the JS Coq package manager.

Note indeed that now all the build artifacts do live in
`_build/default`; `make links` will allow you to serve pages in-tree.

Pending:

- fully split the libmngr build to its own repository.
- libjs: implement recursive scan / get closer to Dune.