Do not need to store class on Dentaku::AST

Repeated addition of many functions will gradually increase the number
of consts attached to Dentaku::AST.  This works just as well if these
remain anonymous classes.

Closes #109

lib/dentaku/ast/function_registry.rb

@@ -36,10 +36,6 @@ def type
           end
         end
 
-        function_class = name.to_s.capitalize.gsub('!', '_')
-        Dentaku::AST.send(:remove_const, function_class) if Dentaku::AST.const_defined?(function_class, false)
-        Dentaku::AST.const_set(function_class, function)
-
         function.implementation = implementation
         function.type = type
 

spec/external_function_spec.rb

@@ -62,10 +62,11 @@
     it 'does not store functions across all calculators' do
       calculator1 = Dentaku::Calculator.new
       calculator1.add_function(:my_function, :numeric, ->(x) { 2*x + 1 })
-      expect(calculator1.evaluate("1 + my_function(2)")). to eq (1 + 2*2 + 1)
 
       calculator2 = Dentaku::Calculator.new
       calculator2.add_function(:my_function, :numeric, ->(x) { 4*x + 3 })
+
+      expect(calculator1.evaluate("1 + my_function(2)")). to eq (1 + 2*2 + 1)
       expect(calculator2.evaluate("1 + my_function(2)")). to eq (1 + 4*2 + 3)
 
       expect{Dentaku::Calculator.new.evaluate("1 + my_function(2)")}.to raise_error(Dentaku::ParseError)