[IMP] fields: faster record.id access

This is a partial backport of revision
ed32700925132943fc4957ae2f43deaeeec7a278

to speed up the access of browse record id field.

odoo/fields.py

@@ -2528,9 +2528,15 @@ class Id(Field):
     def __get__(self, record, owner):
         if record is None:
             return self         # the field is accessed through the class owner
-        if not record:
+
+        # the code below is written to make record.id as quick as possible
+        ids = record._ids
+        size = len(ids)
+        if size is 0:
             return False
-        return record.ensure_one()._ids[0]
+        elif size is 1:
+            return ids[0]
+        raise ValueError("Expected singleton: %s" % record)
 
     def __set__(self, record, value):
         raise TypeError("field 'id' cannot be assigned")