Added the possibility to verify subproofs to the new prover

An assertion feature can now have the form

     all(a,b,...)
         require
	     ...
         check
             ...
             all(x,y) require ... check ... ensure ... end
             require ... check ... ensure ... end
             ...
         ensure
             ...
         end

	modified:   ocaml/context.ml
	modified:   ocaml/proof_context.ml
	modified:   ocaml/proof_table.ml
	modified:   ocaml/prover2.ml