Tf-Ncl contracts are too monolithic

[vkleen]

Schema contracts are huge, monotlithic records at the moment. For example, the AWS Nickel contract is a whopping 30M of code. This quickly becomes unmanageable and slow.

Ideally, these contracts should be built up out of modular pieces, say one generic contract for Terraform builtins and then one contract per provider resource. These should be importable on a piecemeal basis so that tf-ncl library authors can import just those resource contracts that they need.