Add "pretty" subcommand

src/pretty.rs

@@ -0,0 +1,9 @@
+use pretty::{DocAllocator, DocBuilder, Pretty};
+
+use crate::ast::Program;
+
+impl<'a, A: DocAllocator<'a>> Pretty<'a, A> for Program {
+    fn pretty(self, allocator: &'a A) -> DocBuilder<'a, A, ()> {
+        allocator.text("Hello world")
+    }
+}