Deprecate misc instant/*.vim files

Should we get rid of maktaba's feature to detect and source vimscript files under a plugin's instant/ directory? It's one of the more "magical" features and contributes to slow startup times.

There's one special case of instant/flags.vim tracked separately in #189 that deserves some special attention. For other cases, if "instant" setup really can't be deferred until later in vim's init process, it could just go into an autoload helper function designed to be called explicitly in vimrc.