Added a few more test cases for CustomizeDiff, caught another error in
the process. I think this is ready for review now, and possibly some
real-world testing of the waters by way of porting some resources that
would benefit from the feature.
This folder contains helper libraries for Terraform plugins. A running
joke is that this is "Terraform standard library" for plugins. The goal
of the packages in this directory are to provide high-level helpers to
make it easier to implement the various aspects of writing a plugin for
Terraform.