Move eln files into dedicated cache directories

Move eln files into dedicated cache directories

When loading a elc file search for a corresponding eln one into
`comp-eln-load-path' directories and load it if available.
`comp-eln-load-path' contains by default two directory (user and
system one).

	* src/pdumper.c (dump_do_dump_relocation): While resurrecting from
	load set eln cache sys dir in `Vcomp_eln_load_path'.

	* src/lread.c (maybe_swap_for_eln): New function.
	(Fload): Clean-up some now unnecessary code going
	back to the master one.
	(Fload): Make use of Vcomp_eln_to_el_h for the reverse file
	look-up.
	(openp_add_middle_dir_to_suffixes)
	(openp_max_middledir_and_suffix_len, openp_fill_filename_buffer):
	Remove functions.
	(openp): As for Fload revert code modifications.
	(openp): When a .elc file is being loaded check if a corresponding
	eln can be loaded in place.

	* src/comp.c (ELN_FILENAME_HASH_LEN): New macro.
	(comp_hash_string): New function.
	(hash_native_abi): Make use of 'comp_hash_string'.
	(hash_native_abi): Change `comp-native-path-postfix' format.
	(Fcomp_el_to_eln_filename): New function.
	(Fcomp__compile_ctxt_to_file): Have file_name as a input.
	(Vcomp_eln_to_el_h, Vcomp_eln_load_path): New global varaibles.

	* lisp/startup.el (normal-top-level): Add user eln cache directory
	in `comp-eln-load-path'.

	* lisp/help-fns.el (find-lisp-object-file-name): Reverse look-up
	files using `comp-eln-to-el-h'.

	* lisp/files.el (locate-file): Likewise.

	* lisp/emacs-lisp/find-func.el (find-library-name): Likewise.

	* lisp/emacs-lisp/comp.el (comp-output-directory)
	(comp-output-base-filename, comp-output-filename): Remove function.
	(comp-compile-ctxt-to-file): Create parent directories if
	necessary.
	(comp-run-async-workers, native-compile, native-compile-async):
	Make use `comp-el-to-eln-filename'.