Allow punning in record updates

[heueristik]

Currently, punning is possible

Resource.mk@{
  logic;
  label;
  quantity;
  data;
  eph;
  npk;
  nonce;
  rseed
};

whereas it is not possible when a record is copied and edited in place

r@Resource{
  eph; -- instead, you have to do `eph := ephemeral` or similar
};

which would be nice to have.