A unique hash of the BigInt's value suitable for use in a hash table.
toHash is rarely directly invoked; it is implicitly used when BigInt is used as the key of an associative array.
string[BigInt] aa; aa[BigInt(123)] = "abc"; aa[BigInt(456)] = "def"; assert(aa[BigInt(123)] == "abc"); assert(aa[BigInt(456)] == "def");