Implement unify/compare of tuples in Mercury.

The hand-written C unify and compare predicates for tuples did not preserve
deep profiler invariants correctly across the recursive unify/compare of
tuple arguments.  I tried to do so, and failed.  Instead, implement the
predicates in Mercury so the compiler can perform the deep profiling
transformation on them.  Bug #3.

A micro-benchmark on my machine is about twice as fast in asm_fast.gc
after this patch, and about the same in hlc.gc.  The change to the
high-level C backend is only to reduce code duplication.

library/library.m:
	Add foreign-exported predicates to take the place of `io.init_state'
	and `io.finalize_state' as overall initialization/finalization
	predicates.

library/builtin.m:
	Add `unify_tuple', `compare_tuple', `compare_rep_tuple' predicates.

	Add initializer which sets `MR_special_pred_hooks' to point to those
	predicates.

	Delete unused predicates `call_rtti_generic_unify',
	`call_rtti_generic_compare'.

	Reorder some code.

library/io.m:
	Add `io.finalize_state' to hidden interface.

	Delete foreign exports for `io.init_state' and `io.finalize_state'.

	Reorder some code.

runtime/mercury_ho_call.c:
runtime/mercury_ho_call.h:
	Add a global variable `MR_special_pred_hooks' for the library to
	set up during initialisation.

	Add `tailcall' macros for use by `mercury_unify_compare_body.h'.
	Rename `tailcall_user_pred' to `tailcall_tci_pred'.

	Call the new unify/compare predicates in the high-level C
	backend via `MR_special_pred_hooks'.

	Delete `unify_tuples' and `compare_tuples' for the high-level C
	and call the Mercury predicates set in `MR_special_pred_hooks'.

runtime/mercury_unify_compare_body.h:
	Delete the unify and compare code for tuples in the low-level C
	backend.  Jump to the Mercury predicates set in
	`MR_special_pred_hooks' instead.

	Add some comments.

compiler/elds_to_erlang.m:
runtime/mercury_init.h:
runtime/mercury_wrapper.c:
util/mkinit.c:
	Conform to the library initializer/finalizer change.

tests/hard_coded/Mmakefile:
tests/hard_coded/tuple_test2.exp
tests/hard_coded/tuple_test2.m:
	Add test case.

NEWS:
	Announce the change.