Improved separate compilation

[vouillon]

• A single wasm file is generated for each cma archive, which avoid having to load too many module.
• Support linking wasmo files into one wasma file. One can do:

  wasm_of_ocaml a.cmo
  wasm_of_ocaml b.cmo
  wasm_of_ocaml link -a -o foo.wasma a.wasmo b.wasmo
  

  instead of:

  ocamlc -a foo.cma a.cmo b.cmo
  wasm_of_ocaml link -o foo.wasma foo.cma
  

  This allows to make separate compilation more fine-grained, only regenerating Wasml code for modified files.
  See Improve incrementality of jsoo build ocaml/dune#7389