Fix NatSpec style comments of functions

[nrryuya]

@public
@payable
def doesEat(food: string):
  """
  @author Bob Clampett
  @notice Determine if Bugs will accept `food` to eat
  @dev Compares the entire string and does not rely on a hash
  @param food The name of a food to evaluate (in English)
  @return true if Bugs will eat it, false otherwise
  """

  // ...

https://github.com/ethereum/vyper/blob/4da7adf69e880902a5d702754a39d2100cb9bde5/docs/structure-of-a-contract.rst#natspec-metadata

Note: other refactors

• use is_contracts in ERC721
• use += and -= in ERC20