This diff changes a few types from being defined as equivalent to a pair

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This diff changes a few types from being defined as equivalent to a pair
to being discriminated union types with their own function symbol. This
was motivated by an error message (one of many, but the one that broke
the camel's back) about "-" being used in an ambiguous manner. It will
reduce the number of such messages in the future, and will make compiler
data structures easier to inspect in the debugger.

The most important type changed by far is hlds_goal, whose function symbol
is now "hlds_goal". Second and third in importance are llds.instruction
(function symbol "llds_instr") and prog_item.m's item_and_context (function
symbol "item_and_context"). There are some others as well.

In several places, I rearranged predicates to factor the deconstruction of
goals into hlds_goal_expr and hlds_goal_into out of each clause into a single
point. In many places, I changed variable names that used "Goal" to refer
to just hlds_goal_exprs to use "GoalExpr" instead. I also changed variable
names that used "Item" to refer to item_and_contexts to use "ItemAndContext"
instead. This should make reading such code less confusing.

I renamed some function symbols and predicates to avoid ambiguities.

I only made one algorithmic change (at least intentionally).
In assertion.m, comparing two goals for equality now ignores goal_infos
for all kinds of goals, whereas previously it ignored them for most kinds
of goals, but for shorthand goals it was insisting on them being equal.
This seemed to me to be a bug. Pete, can you confirm this?

compiler/structure_reuse.direct.detect_garbage.m

@@ -1,7 +1,7 @@
 %-----------------------------------------------------------------------------%
 % vim: ft=mercury ts=4 sw=4 et
 %-----------------------------------------------------------------------------%
-% Copyright (C) 2006 The University of Melbourne.
+% Copyright (C) 2006-2007 The University of Melbourne.
 % This file may only be copied under the terms of the GNU General
 % Public License - see the file COPYING in the Mercury distribution.
 %-----------------------------------------------------------------------------%
@@ -86,7 +86,7 @@ determine_dead_deconstructions(Background, Goal, DeadCellTable):-
 
 determine_dead_deconstructions_2(Background, TopGoal, !SharingAs, 
         !DeadCellTable) :- 
-    TopGoal = GoalExpr - GoalInfo, 
+    TopGoal = hlds_goal(GoalExpr, GoalInfo), 
     ModuleInfo = Background ^ module_info, 
     PredInfo = Background ^ pred_info, 
     ProcInfo = Background ^ proc_info,