Allow compilation with foreign object files

[zbaylin]

Desired Behavior

Per my discussion with @rgrinberg on Discord earlier, as of now in order to include an object file in compilation, it first needs to be embedded in a .a file with ar.

My thought was to add a (foreign_objects <foreign-objects-list>) field that behaves the same way as (foreign_archives <foreign-archives-list>), except with .o files.

Example

dune:

(library
 (name mylib)
 (foreign_objects myobject.o))

(rule
 (targets myobject.o)
 (action ...))

[rgrinberg]

foreign_objects is the name that I would prefer, but in this field is actually more similar to foreign_stubs. It's essentially the same as having (foreign_stubs (language custom) (names myobject)), and then manually setting up the (rule ..) as you did.

Such a custom language wouldn't setup any rules in Foreign_rules.build_o_files, but would still return the object file.

The name foreign_objects is still preferred though.

To implement this feature, start by reading our hacking docs. If there's anything unclear, I'm happy to expand them.

You should familiarize yourself with the modules Foreign, Foreign_rules, and parts of Dune_file.