* IPv6 support

Add variable "enable_ipv6" to allow enabling IPv6 support (resulting in
passing "assign_generated_ipv6_cidr_block" to aws_vpc.

Enabling IPv6 support further results in an Egress-only internet gateway
being provisioned and routing tables of subnets being adjusted.

Additional variables allow to choose the indices out of the /64 subnets
based on the assigned /56 range.

* Add example for IPv6 usage

* Remove redundant parameter assign_generated_ipv6_cidr_block

This is needed exactly when var.enable_ipv6 is true.

* Set subnet ipv6_cidr_block to null if unused

* Be picky about spelling

* Revert unrelated change

* More IPv6 spelling

* Added IPv6 support to VPC module

* Added IPv6 support to VPC module