Merge in the changes from the existential_types_2 branch.

Estimated hours taken: 4

Merge in the changes from the existential_types_2 branch.

This change adds support for mode re-ordering of code involving
existential types.  The change required modifying the order of the
compiler passes so that polymorphism comes before mode analysis,
so that mode analysis can check the modes of the `type_info' or
`typeclass_info' variables that polymorphism introduces, so that
it can thus re-order the code accordingly.

This change also includes some more steps towards making existential data
types work.  In particular, you should be able to declare existentially
typed data types, the compiler will generate appropriate unification
and compare/3 routines for them, and deconstruction unifications for them
should work OK.  However, currently there's no way to construct them
except via `pragam c_code', and we don't generate correct RTTI for them,
so you can't use `io__write' etc. on them.

library/private_builtin.m:
compiler/accumulator.m:
compiler/bytecode_gen.m:
compiler/check_typeclass.m:
compiler/clause_to_proc.m:
compiler/code_util.m:
compiler/common.m:
compiler/dead_proc_elim.m:
compiler/dependency_graph.m:
compiler/det_analysis.m:
compiler/det_report.m:
compiler/follow_code.m:
compiler/follow_vars.m:
compiler/goal_util.m:
compiler/higher_order.m:
compiler/hlds_goal.m:
compiler/hlds_out.m:
compiler/hlds_pred.m:
compiler/intermod.m:
compiler/lambda.m:
compiler/live_vars.m:
compiler/magic.m:
compiler/make_hlds.m:
compiler/mercury_compile.m:
compiler/mercury_to_c.m:
compiler/mode_errors.m:
compiler/mode_info.m:
compiler/mode_util.m:
compiler/modecheck_call.m:
compiler/modecheck_unify.m:
compiler/modes.m:
compiler/pd_cost.m:
compiler/polymorphism.m:
compiler/post_typecheck.m:
compiler/purity.m:
compiler/quantification.m:
compiler/rl_exprn.m:
compiler/rl_key.m:
compiler/simplify.m:
compiler/table_gen.m:
compiler/term_traversal.m:
compiler/type_util.m:
compiler/typecheck.m:
compiler/unify_gen.m:
compiler/unify_proc.m:
compiler/unique_modes.m:
compiler/unused_args.m:
compiler/notes/compiler_design.html:
doc/reference_manual.texi:
tests/hard_coded/typeclasses/Mmakefile:
tests/hard_coded/typeclasses/existential_data_types.m:
tests/hard_coded/typeclasses/existential_data_types.exp:
tests/warnings/simple_code.exp:
tests/hard_coded/Mmakefile:
tests/term/arit_exp.trans_opt_exp:
tests/term/associative.trans_opt_exp:
tests/term/pl5_2_2.trans_opt_exp:
tests/term/vangelder.trans_opt_exp:
tests/term/arit_exp.trans_opt_exp:
tests/term/associative.trans_opt_exp:
tests/term/pl5_2_2.trans_opt_exp:
tests/term/vangelder.trans_opt_exp:
tests/invalid/errors2.err_exp2:
tests/invalid/prog_io_erroneous.err_exp2:
tests/invalid/type_inf_loop.err_exp2:
tests/invalid/types.err_exp2:
tests/invalid/polymorphic_unification.err_exp:
tests/invalid/Mmakefile:
tests/warnings/simple_code.exp:
tests/debugger/queens.exp:
tests/hard_coded/Mmakefile:
tests/hard_coded/existential_reordering.m:
tests/hard_coded/existential_reordering.exp:
	Merge in the changes from the existential_types_2 branch.

compiler/code_util.m

@@ -746,7 +746,7 @@ code_util__cannot_stack_flush(GoalExpr - _) :-
 :- mode code_util__cannot_stack_flush_2(in) is semidet.
 
 code_util__cannot_stack_flush_2(unify(_, _, _, Unify, _)) :-
-	Unify \= complicated_unify(_, _).
+	Unify \= complicated_unify(_, _, _).
 code_util__cannot_stack_flush_2(call(_, _, _, BuiltinState, _, _)) :-
 	BuiltinState = inline_builtin.
 code_util__cannot_stack_flush_2(conj(Goals)) :-