Turn some value_contents functions into methods

This turns value_contents_raw, value_contents_writeable, and
value_contents_all_raw into methods on value.  The remaining functions
will be changed later in the series; they were a bit trickier and so I
didn't include them in this patch.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/c-lang.c

@@ -674,7 +674,7 @@ c_string_operation::evaluate (struct type *expect_type,
 	    error (_("Too many array elements"));
 
 	  result = value::allocate (expect_type);
-	  memcpy (value_contents_raw (result).data (), obstack_base (&output),
+	  memcpy (result->contents_raw ().data (), obstack_base (&output),
 		  obstack_object_size (&output));
 	}
       else