Split equiv_type.m into two parts.

The old equiv_type.m's growth was organic, not planned. It mixed predicates
operating on different levels, and even predicates on the same level
were not always logically organized.

compiler/equiv_type_parse_tree.m:
    This module contains the bulk of the old equiv_type.m, the part
    concerned with processing parse tree items and their major components.

compiler/equiv_type.m:
    The part of the old equiv_type.m that deals with expanding equivalences
    inside basic parts of program representations.

compiler/parse_tree.m:
compiler/notes/compiler_design.html:
    Include and document the new module.

compiler/decide_type_repn.m:
compiler/mercury_compile_make_hlds.m:
    Conform to the changes above.

compiler/decide_type_repn.m

@@ -166,6 +166,7 @@
 :- import_module libs.options.
 :- import_module parse_tree.build_eqv_maps.
 :- import_module parse_tree.equiv_type.
+:- import_module parse_tree.equiv_type_parse_tree.
 :- import_module parse_tree.parse_tree_out_type_repn.
 :- import_module parse_tree.prog_data.
 :- import_module parse_tree.prog_type.

compiler/mercury_compile_make_hlds.m

@@ -61,7 +61,7 @@
 :- import_module mdbcomp.
 :- import_module mdbcomp.sym_name.
 :- import_module parse_tree.build_eqv_maps.
-:- import_module parse_tree.equiv_type.
+:- import_module parse_tree.equiv_type_parse_tree.
 :- import_module parse_tree.error_util.
 :- import_module parse_tree.file_names.
 :- import_module parse_tree.generate_mmakefile_fragments.

compiler/notes/compiler_design.html

@@ -698,17 +698,22 @@ are module qualified during mode analysis.
 </ul>
 
 <p>
-The parse tree module also contains equiv_type.m,
-which does expansion of equivalence types,
-and of `with_type` and `with_inst` annotations
-on predicate and function type and mode declarations.
+The parse tree module also contains equiv_type_parse_tree.m,
+which orchestrates
+the expansion of equivalence types in all parse tree components,
+and also propagates the information
+in `with_type` and `with_inst` annotations
+into the type and mode declarations of predicates and functions.
 The part of its job gathering the definitions to be expanded
-is done by build_eqv_maps.m.
+is done by build_eqv_maps.m,
+and the part involving expansions in basic parts of program representations
+(meaning, those significantly smaller than a prog_item)
+is done by equiv_type.m.
 <p>
 Expansion of equivalence types is really part of type-checking,
 but is done on the parse tree rather than on the HLDS
 because it turned out to be much easier to implement that way.
-(Though later we had to add a pass the expand equivalence types
+(Though later we had to add a pass to expand equivalence types
 in the HLDS anyway.)
 
 <h4>The hlds.m package</h4>

compiler/parse_tree.m

@@ -115,6 +115,7 @@
 :- include_module convert_parse_tree.
 :- include_module deps_map.
 :- include_module equiv_type.
+:- include_module equiv_type_parse_tree.
 :- include_module file_names.
 :- include_module get_dependencies.
 :- include_module grab_modules.