The revision field is only populated on dev builds so this means
most releases of Terraform have an empty "terraform_revision" field
in the JSON output. Since we recommend developers use go tooling
to `go build` this tool when developing, the revision is not useful
data and so it is removed.
* lang/funcs: testing of functions through the lang package API
The function-specific unit tests do not cover the HCL conversion that happens when the functions are called in a terraform configuration. For e.g., HCL converts sets to lists before passing it to the function. This means that we could not test passing a set in the function _unit_ tests.
This adds a higher-level acceptance test, plus a check that every (pure) function has a test.
* website/docs: update function documentation
