Simplify the build with vendoring

[djs55]

The current build is complex because it relies on

• opam for package selection and installation
• the network for downloading packages
• Cygwin on windows for build scripts
• out of date dependency versions

In particular the Windows CI is broken due to a Cygwin bug.

The build could be simplified if we switched to a vendor model with opam monorepo. We would be able to build by git clone and dune build (assuming an OCaml compiler is installed)

To achieve this we need to

• update our DNS dependencies to use dune and vendor: Vendor the OCaml DNS code #542
• switch from Uwt to Luv for libuv: Replace Uwt with Luv #539
• update to the latest TCP/IP stack interfaces
• initialise the monorepo