Disallow ambiguous conversions between number literals and bytesXX types.

docs/abi-spec.rst

@@ -444,7 +444,7 @@ For example,
     pragma solidity >0.4.24;
 
     contract Test {
-      constructor() public { b = 0x12345678901234567890123456789012; }
+      constructor() public { b = hex"12345678901234567890123456789012"; }
       event Event(uint indexed a, bytes32 b);
       event Event2(uint indexed a, bytes32 b);
       function foo(uint a) public { emit Event(a, b); }