Only expose signature and holder to SOM language level

The other elements such as number of locals, maximum number of stack elements, and number of method fields are not available on all SOM VMs.

These information should be made available by a VM-specific mirror object.
Signed-off-by: Stefan Marr <git@stefan-marr.de>

src/som/vmobjects/method.py

@@ -7,11 +7,9 @@ from som.interpreter.bytecodes import bytecode_length
 class Method(Array):
     
     # Static field indices and number of method fields
-    NUMBER_OF_LOCALS_INDEX                 = 1 + Array.CLASS_INDEX
-    MAXIMUM_NUMBER_OF_STACK_ELEMENTS_INDEX = 1 + NUMBER_OF_LOCALS_INDEX
-    SIGNATURE_INDEX                        = 1 + MAXIMUM_NUMBER_OF_STACK_ELEMENTS_INDEX
-    HOLDER_INDEX                           = 1 + SIGNATURE_INDEX
-    NUMBER_OF_METHOD_FIELDS                = 1 + HOLDER_INDEX
+    SIGNATURE_INDEX                 = 1 + Array.CLASS_INDEX
+    HOLDER_INDEX                    = 1 + SIGNATURE_INDEX
+    NUMBER_OF_METHOD_FIELDS         = 1 + HOLDER_INDEX
 
     
     def __init__(self, nilObject):
@@ -26,6 +24,9 @@ class Method(Array):
         self._bytecodes              = None
         self._inline_cache_class     = None
         self._inline_cache_invokable = None
+        
+        self._number_of_locals       = -1
+        self._maximum_number_of_stack_elements = -1
     
     def is_primitive(self):
         return False
@@ -38,19 +39,19 @@ class Method(Array):
   
     def get_number_of_locals(self):
         # Get the number of locals (converted to a Java integer)
-        return self.get_field(self.NUMBER_OF_LOCALS_INDEX)
+        return self._number_of_locals
 
     def set_number_of_locals(self, value):
         # Set the number of locals
-        self.set_field(self.NUMBER_OF_LOCALS_INDEX, value)
+        self._number_of_locals = value
 
     def get_maximum_number_of_stack_elements(self):
         # Get the maximum number of stack elements
-        return self.get_field(self.MAXIMUM_NUMBER_OF_STACK_ELEMENTS_INDEX)
+        return self._maximum_number_of_stack_elements
   
     def set_maximum_number_of_stack_elements(self, value):
         # Set the maximum number of stack elements
-        self.set_field(self.MAXIMUM_NUMBER_OF_STACK_ELEMENTS_INDEX, value)
+        self._maximum_number_of_stack_elements = value
 
     def get_signature(self):
         # Get the signature of this method by reading the field with signature