Support melange

[andersfugmann]

Melange requires dune libraries to have (modes melange). However ptime and base64 does not support melange. Until melange support is added upstream, a solution would be to case on melange compilation and provide alternative (or dummy) implementations for the libraries when compiling for melange.

[anmonteiro]

this could be a good use case for dune universal libraries after they come out in 3.16 https://melange.re/blog/posts/dune-universal-libraries-preview

[andersfugmann]

Looks like exactly what we need for this! Great work. I'll hold off implementation until the dune 3.16 is released and leverage this.