Remove some evals from formula DSL

The default arguments were removed from these methods long ago, so
nothing is standing in the way of using define_method here.

Library/Homebrew/formula.rb

@@ -633,12 +633,8 @@ class Formula
       stable.mirror(val)
     end
 
-    Checksum::TYPES.each do |cksum|
-      class_eval <<-EOS, __FILE__, __LINE__ + 1
-        def #{cksum}(val)
-          stable.#{cksum}(val)
-        end
-      EOS
+    Checksum::TYPES.each do |type|
+      define_method(type) { |val| stable.send(type, val) }
     end
 
     def bottle *, &block

Library/Homebrew/resource.rb

@@ -95,12 +95,8 @@ class Resource
     puts "For your reference the SHA1 is: #{fn.sha1}"
   end
 
-  Checksum::TYPES.each do |cksum|
-    class_eval <<-EOS, __FILE__, __LINE__ + 1
-      def #{cksum}(val)
-        @checksum = Checksum.new(:#{cksum}, val)
-      end
-    EOS
+  Checksum::TYPES.each do |type|
+    define_method(type) { |val| @checksum = Checksum.new(type, val) }
   end
 
   def url val=nil, specs={}