2.4.0

CHANGES:

• minor performance fixes (#53 by freddy77)
• use dune 2.0 (#54 by lindig)
• require OCaml 4.13 (#56 by last-genius)
• Add support XS_DIRECTORY_PART to the client (#55 by last-genius)
• Add x-maintenance-intent to opam metadata (#57 by hannesm)