#4 Problem on 64-bit machine

[Simon Wright]

On a 64-bit machine (x64_64-apple-darwin12, for example) System.Word_Size is 64, so that at crypto-types-big_numbers.ads:324 Max_Length is 0, so that when test-elliptic_curves.adb instantiates Crypto.Types.Big_Numbers (Size => 64) there is an unavoidable Constraint_Error at crypto-types-big_numbers-utils.adb:126 and other places.