* lib/Automake/Item.pm, lib/Automake/ItemDef.pm: New files.

* lib/Automake/Item.pm, lib/Automake/ItemDef.pm: New files.
* lib/Automake/Rule.pm, lib/Automake/RuleDef.pm: New files.
* lib/Automake/Makefile.am (dist_perllib_DATA): Add them.
* lib/Automake/VarDef.pm: Make this a subclass of Automake::ItemDef.
(new): Adjust to call Automake::ItemDef::new.
(comment, location, owner): Delete.  Now inherited from ItemDef.
* lib/Automake/Variable.pm: Make this a subclass of Automake::Item.
(_new): Adjust to call Automake::Item::new.
(name, def, rdef, _set, conditions, not_always_defined_in_cond):
Delete.  How inherited from Item, where `_set' is called `set'.
* automake.in (SUFFIX_RULE_PATTERN): Delete. Now in Automake::Rule.
(suffix_rules_default): Delete.  Now
Automake::Rule::_suffix_rules_default
(suffixes): Delete.  Now Automake::Rule::suffixes.
(TARGET_AUTOMAKE, TARGET_USER): Delete.  Now
Automake::RuleDef::RULE_AUTOMAKE and Automake::RuleDef::RULE_USER.
(%targets, %target_source, %target_name, %target_owner): Delete,
replaced by the Rule and RuleDef classes.
(dependencies, depend, actions): Delete.  Now in Automake::Rule.
(suffix_rules, register_suffix_rule): Likewise.
(KNOWN_EXTENSIONS_PATTERN, accept_extensions): Likewise.
(known_extensions_list): Delete.  Now
Automake::Rule::_known_extensions_list.
(target_conditions): Delete.  Now inherited by Automake::Rule
from Automake::Item::conditions.
(rule_define): Delete.  Now Automake::Rule::define.  Adjust all
callers.
(target_defined): Delete.  Now Automake::Rule::rule.  Adjust all
callers.
(initialize_per_input): Adjust to call Automake::Rule::reset.
(err_target, err_cond_target, msg_cond_target, msg_target,
reject_target): Delete.  Now defined in Automake::Rule as
err_rule, err_cond_rule, msg_cond_rule, msg_rule and reject_target.
Adjust all callers.
(handle_languages): Call suffix_rules_count.
* tests/location.test: Adjust expected diagnostics.  We now display
$(EXEEXT) accurately.