cut toploop(s) out of coqtop: now they are loaded dynamically

User interface writers can drop a footop.cmxs in $(COQLIB)/toploop/
and pass -toploop footop to customize the coq main loop.

A toploop must set Coqtop.toploop_init and Coqtop.toploop_run to
functions respectively initializing the toploop (and parsing toploop
specific arguments) and running the main loop itself.

For backward compatibility -ideslave and -async-proofs worker do
set the toploop to coqidetop and stmworkertop respectively.