Extracting the mom instruction from builtin modules

Since they were embedded at first (easier copy/paste) they now got own files, like their brethren
also mini tests for each instruction , nice start

lib/mom/builtin/exit.rb

@@ -0,0 +1,12 @@
+module Mom
+  module Builtin
+    class Exit < ::Mom::Instruction
+      def to_risc(compiler)
+        builder = compiler.builder(compiler.source)
+        builder.prepare_int_return # makes integer_tmp variable as return
+        Builtin.exit_sequence(builder)
+        return compiler
+      end
+    end
+  end
+end