[archi] put Visit' submodules in independent files

src/plugins/e-acsl/Makefile.in

src/plugins/e-acsl/src/code_generator/global_observer.ml

src/plugins/e-acsl/src/code_generator/global_observer.mli

src/plugins/e-acsl/src/code_generator/literal_observer.ml

src/plugins/e-acsl/src/code_generator/literal_observer.mli

src/plugins/e-acsl/src/code_generator/memory_observer.ml

src/plugins/e-acsl/src/code_generator/memory_observer.mli

src/plugins/e-acsl/src/code_generator/visit.ml