Various changes in the context tables to be a good infrastructure for the prover

	modified:   ocaml/context.ml
	modified:   ocaml/context.mli
	modified:   ocaml/proof_context.ml
	modified:   ocaml/proof_context.mli
	modified:   ocaml/proof_table.ml
	modified:   ocaml/proof_table.mli
	modified:   ocaml/term.ml
	modified:   ocaml/typer.ml