Make rupro folded class available to OCaml
Summary:
Introduce an FFI that takes in a source file and applies the rupro folding logic to produce the folded class in that file along with the class name, returning the values in OCaml
```
external decl_folded_class_in_file :
Relative_path.t -> (Decl_defs.decl_class_type * string) option
= "decl_folded_class_in_file_ffi"
```
Also refactor out some common utilities.
Differential Revision:
D34605827
fbshipit-source-id:
3f555b10d1324e239c938569d632a78323e7a3eb