Ethernet shield and IPv6 support
There is no library, and there physically cannot be one. IPv4 is built into the Ethernet shield itself. (It has a Wiznet 510 chip.)


  Shields based on the MicroChip ENC28J60 chip (such as those sold by Ekitszone and Nuelectronics) are suitable for the implementation of an IPv6 or dual IPv4/IPv6 stack. This approach requires the TCP state machine to be implemented in software and results in more complex arduino code.


Source: this tutorial. (You may find it useful, I'd recommenced reading it.)

There is this library for IPv6 on other hardware, not for the official Ethernet shield.