Make autoloads populate a new definition-prefixes table
* lisp/subr.el (definition-prefixes): New hash table.
(register-definition-prefixes): New function.
* lisp/emacs-lisp/autoload.el (autoload-compute-prefixes): New var.
(autoload--split-prefixes-1, autoload--split-prefixes)
(autoload--make-defs-autoload): New functions.
(autoload-defs-autoload-max-size, autoload-popular-prefixes): New vars.
(autoload-generate-file-autoloads): Obey autoload-compute-prefixes.
(update-directory-autoloads): Don't touch loaddefs.el if the set of
autoloads hasn't changed (i.e. if only the timestamp would change).
* lisp/loadup.el: Purify definition-prefixes.
* lisp/w32-fns.el: Keep name space clean.
(w32-set-default-process-coding-system): Rename from
set-default-process-coding-system.
(w32-set-system-coding-system): Rename from set-w32-system-coding-system.