Lazily initialize MethodEntity collections

MethodEntity#initialize was allocating 4 Sets and 1 Hash eagerly.
Use lazy accessor methods to defer allocation until first use.
Most MethodEntity instances are created during module resolution
but many fields are never populated.

Measured with tool/dog_bench.rb (stackprof, cumulative with prior commits):
- GC overhead: 28.6% -> 23.7%
- Total samples: 1038 -> 1001

Author: sinsoku

lib/typeprof/core/env/method_entity.rb

@@ -2,60 +2,63 @@ module TypeProf::Core
   class MethodEntity
     def initialize
       @builtin = nil
-      @decls = Set.empty
-      @overloading_decls = Set.empty
-      @defs = Set.empty
-      @aliases = {}
-      @method_call_boxes = Set.empty
     end
 
-    attr_reader :decls, :defs, :aliases, :method_call_boxes
     attr_accessor :builtin
 
+    def decls = @decls ||= Set.empty
+    def defs = @defs ||= Set.empty
+    def aliases = @aliases ||= {}
+    def method_call_boxes = @method_call_boxes ||= Set.empty
+
+    private def overloading_decls = @overloading_decls ||= Set.empty
+
     def add_decl(decl)
       if decl.overloading
-        @overloading_decls << decl
+        overloading_decls << decl
       else
-        @decls << decl
+        decls << decl
       end
     end
 
     def remove_decl(decl)
       if decl.overloading
-        @overloading_decls.delete(decl) || raise
+        overloading_decls.delete(decl) || raise
       else
-        @decls.delete(decl) || raise
+        decls.delete(decl) || raise
       end
     end
 
     def add_def(mdef)
-      @defs << mdef
+      defs << mdef
       self
     end
 
     def remove_def(mdef)
-      @defs.delete(mdef) || raise
+      defs.delete(mdef) || raise
     end
 
     def add_alias(node, old_mid)
-      @aliases[node] = old_mid
+      aliases[node] = old_mid
     end
 
     def remove_alias(node)
-      @aliases.delete(node) || raise
+      aliases.delete(node) || raise
     end
 
     def exist?
-      @builtin || !@decls.empty? || !@defs.empty?
+      @builtin || (@decls && !@decls.empty?) || (@defs && !@defs.empty?)
     end
 
     def add_run_all_mdefs(genv)
+      return unless @defs
       @defs.each do |mdef|
         genv.add_run(mdef)
       end
     end
 
     def add_run_all_method_call_boxes(genv)
+      return unless @method_call_boxes
       @method_call_boxes.each do |box|
         genv.add_run(box)
       end