Add 'cddl' module providing Haskell bindings to Rust's cddl crate.

  This is meant to be used in test modules, for validating conformance
  of CBOR-serialized data objects against some CDDL specification.

  Note that ouroboros-network is doing something similar, but using a
  Ruby gem and calling directly the command-line built from the gem. We
  could do similar since the Rust cddl crate also has a CLI.

  The Haskell <-> Rust FFI is a bit of an experiment here as it is meant
  to provide a better (and faster) API than repeated IO calling a
  command-line in a shell.

cabal.project

@@ -26,6 +26,7 @@ packages:
     cardano-submit-api
     cardano-testnet
     cardano-tracer
+    cddl
     bench/cardano-topology
     bench/locli
     bench/plutus-scripts-bench

cddl/.gitignore

@@ -0,0 +1,15 @@
+### Rust ###
+# Generated by Cargo
+# will have compiled files and executables
+debug/
+target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk
+
+# MSVC Windows builds of rustc generate these, which store debugging information
+*.pdb