// function does not verify the contents of each field.
//
// This function performs two transformations:
//   - The last 32 bits of an IPv6 address may be represented in
//     IPv4-style dotted quad form, as in 1:2:3:4:5:6:7.8.9.10. That
//     address is transformed to its hex equivalent,
//     e.g. 1:2:3:4:5:6:708:90a.
//   - An address may contain one "::", which expands into as many