Carve four new modules out of prog_item.m.

compiler/prog_item_type.m:
    New module containing the parts of prog_item.m dealing with
    type definitions.

compiler/prog_item_inst_mode.m:
    New module containing the parts of prog_item.m dealing with
    insts and mode definitions.

compiler/prog_item_pragma.m:
    New module containing the parts of prog_item.m dealing with
    pragmas.

compiler/prog_item_pred_proc_id.m:
    New module containing the parts of prog_item.m dealing with
    identifying predicates and procedures (by name, not by number).

compiler/prog_item.m:
    Delete the moved code. Also delete an unused type.

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

configure.ac:
    Require the installed compiler to NOT check subtype definitions
    that occur in .int2 files. Without this fix, the compilation
    of many modules in the compiler will incorrectly report the
    following error, due to the definitions of the two mentioned types
    from prog_item_pred_proc_id.m not being visible:

    parse_tree.prog_item_pragma.int2:216: Error: the second argument of
    parse_tree.prog_item_pragma.int2:216:   `item_decl_marker_info' has a type,
    parse_tree.prog_item_pragma.int2:216:   `parse_tree.prog_item_pred_proc_id.pred_pfu_name_arity_pf',
    parse_tree.prog_item_pragma.int2:216:   which is not a subtype of the
    parse_tree.prog_item_pragma.int2:216:   corresponding argument type
    parse_tree.prog_item_pragma.int2:216:   `parse_tree.prog_item_pred_proc_id.pred_pfu_name_arity'
    parse_tree.prog_item_pragma.int2:216:   in the supertype.

compiler/*.m:
    Adjust imports as needed.