Slither fails to detect some constants

The Echidna printer in Slither fails to detect some constants, for instance:

```solidity
contract C {
uint x = 1;
}
```

won't be detected:

```
$ slither --print echidna small.sol
...
INFO:Printers:{
"payable": {},
"timestamp": {},
"block_number": {},
"msg_sender": {},
"msg_gas": {},
"assert": {},
"constant_functions": {},
"constants_used": {},
"constants_used_in_binary": {},
"functions_relations": {
"C": {}
},
"constructors": {},
"have_external_calls": {},
"call_a_parameter": {},
"use_balance": {}
}
```