The field reader code path is extremely inefficient, but refactoring
it all is much to invasive a change at the moment.
Have DiffFieldReader internally cache results for ReadField.
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.