Make the library runnable in bytecode

[vthemelis]

If I try to run the executable of this library as bytecode, I get the following:

◦ dune exec ./parse_and_print.bc 
File "dune", line 35, characters 7-22:
35 |  (name parse_and_print)
            ^^^^^^^^^^^^^^^
Error: No rule found for dllffi_ocaml.so

Could you add a dynamic library build as well?

I'm interested in this to allow running this and pyre in the debugger.