Support putting all tooling binaries into shell environment

As per tool management requirements 2.2. Shells, 2.3. Programmatic use, 3.2.2.1. Usable as tools, and 3.2.3.1. Usable as tools, it should be possible to put all tools managed by dune in the executable lookup path for a user.

IMO, this would ideally be via a single directory of links to binaries, which could them be added as, e.g., PATH=_build/tools/bin:$PATH. This follows a common convention across package and project management tooling, avoids cluttering the path, and avoids the problem of stale PATH entries that comes from adding a path for each installed binary individually. It does require a mechanism to keep the bin directory in sync, but I would expect this to be relatively straight forward to address via a bimap.

Start with the tool-management requirements in doc/dev/tools-design/requirements.md, especially sections 2.2, 2.3, 3.2.2.1, and 3.2.3.1. Trace how Dune manages tools and determine how a shared executable directory could stay synchronized; done means users can add that directory to PATH and access all managed tool binaries.