First version of 'Proof_context' added

The proof context is already part of the context, but participates
up to now only in the push and pop operations.

	modified:   ocaml/context.ml
	modified:   ocaml/feature_table.ml
	modified:   ocaml/feature_table.mli
	new file:   ocaml/proof_context.ml
	new file:   ocaml/proof_context.mli