ZERO_WEI constant can be defined with no effect

[smarx]

ZERO_WEI is a built-in constant, so it shouldn't be allowed as a user-defined constant:

ZERO_WEI: constant(uint256(wei)) = 3
ONE_WEI: constant(uint256(wei)) = 5

@public
@constant
def zero() -> uint256(wei):
    return ZERO_WEI # returns 0, not 3

@public
@constant
def one() -> uint256(wei):
    return ONE_WEI # returns 5, as expected

[fubuloubu]

Easy fix: disallow built-in constants from userland

Eventually consider namespacing? (Via #1347)