lang/dhall: import dhall-1.40.2

Dhall is an explicitly typed configuration language that is not Turing
complete. Despite being Turing incomplete, Dhall is a real programming
language with a type-checker and evaluator.

Use this library to parse, type-check, evaluate, and pretty-print the Dhall
configuration language. This package also includes an executable which
type-checks a Dhall file and reduces the file to a fully evaluated normal
form.

(pho)