cache index resolution

resolve once and reuse
also use non checking version of get/set
as index per definition must be ok
This creates some speedup, but mostly avoids some weird hang in Thread::Queue.pop for many seconds

lib/risc/fake_memory.rb

@@ -18,17 +18,23 @@ module Risc
     end
     def set(index , value)
       range_check(index)
+      _set(index,value)
+    end
+    alias :[]=  :set
+    def _set(index , value)
       @memory[index] = value
       value
     end
-    alias :[]=  :set
-
     def get(index)
       range_check(index)
-      @memory[index]
+      _get(index)
     end
     alias :[] :get
 
+    def _get(index)
+      @memory[index]
+    end
+
     def size
       @memory.length
     end