elements: Move from a global is_elements to a global chainparams

We now have a pointer to chainparams, that fails valgrind if we do anything
chain-specific before setting it.

Suggested-by: Rusty Russell <@rustyrussell>

tools/generate-wire.py

@@ -115,9 +115,12 @@ class Field(object):
         return ', const {} *{}'.format(type_name, self.name)
 
     def arg_desc_from(self):
+        type_name = self.type_obj.type_name()
+        if self.type_obj.is_const_ptr_ptr_type():
+            return ', const {} **{}'.format(type_name, self.name)
+
         if self.len_field_of:
             return ''
-        type_name = self.type_obj.type_name()
         if self.is_array():
             return ', {} {}[{}]'.format(type_name, self.name, self.count)
         ptrs = '*'
@@ -244,6 +247,12 @@ class Type(FieldSet):
         # FIXME: omits 'pad'
     }
 
+    # Types that are const pointer-to-pointers, such as chainparams, i.e.,
+    # they set a reference to some const entry.
+    const_ptr_ptr_types = [
+        'chainparams'
+    ]
+
     @staticmethod
     def true_type(type_name, field_name=None, outer_name=None):
         """ Returns 'true' type of a given type and a flag if
@@ -308,6 +317,9 @@ class Type(FieldSet):
     def is_subtype(self):
         return bool(self.fields)
 
+    def is_const_ptr_ptr_type(self):
+        return self.name in self.const_ptr_ptr_types
+
     def is_truncated(self):
         return self.name in self.truncated_typedefs