Cyclic dependency core <-> frontend

[cknitt]

jscomp/frontend cannot be compiled without jscomp/core, and jscomp/core cannot be compiled without jscomp/frontend.

Therefore the core dune file currently has:

(copy_files# ../frontend/*.{ml,mli})

When trying to build independently, the following errors occur:

core:

File "jscomp/core/lam_primitive.mli", line 44, characters 18-42:
Error: Unbound module External_arg_spec
File "jscomp/core/lam_compile_const.mli", line 29, characters 24-45:
Error: Unbound module External_arg_spec
File "jscomp/core/lam_compile_external_obj.mli", line 34, characters 2-30:
Error: Unbound module External_arg_spec
File "jscomp/core/lam_constant_convert.ml", line 32, characters 22-62:
Error: Unbound module Ast_utf8_string_interp

frontend:

File "jscomp/frontend/ast_payload.mli", line 43, characters 7-27:
Error: Unbound module Js_raw_info
File "jscomp/frontend/ast_utf8_string_interp.mli", line 61, characters 45-52:
Error: Unbound module J
File "jscomp/frontend/classify_function.mli", line 25, characters 52-67:
Error: Unbound module Js_raw_info