Added support for 64 bit CRC

Support for both ISO and ECMA 64 bit CRC was added to $(MREF std, digest, crc).

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import std.digest.crc;

void main()
{
    ubyte[8] hash64ecma = crc64ECMAOf("abc");
    assert(crcHexString(hash64ecma) == "2CD8094A1A277627");

    ubyte[8] hash64iso = crc64ISOOf("abc");
    assert(crcHexString(hash64iso) == "3776C42000000000");
}
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