GOOPS needs the observer protocol specified for the new module

GOOPS needs the observer protocol specified for the new module
system.  Here's a simple version for the old module system:
* boot-9.scm (module-observers, module-weak-observers,
module-observer-id, set-module-observers!,
set-module-observer-id!): New accessors.
(module-type): Added slots `observers',	`weak-observers' and
`observer-id'.
(module-observe, module-observe-weak, module-unobserve,
module-modified!): New procedures.
(module-make-local-var!, module-add!, module-remove!,
module-clear!, module-define!, module-use!): Call module-modified!.