allow public uses of structs; add sample program

src/palette.rs

@@ -2,9 +2,9 @@ use crate::colour::Colour;
 
 #[derive(Debug, Eq, PartialEq)]
 pub struct Palette {
-    pub(crate) id: String, // base36 string (why??)
-    pub(crate) name: Option<String>,
-    pub(crate) colours: Vec<Colour>,
+    pub id: String, // base36 string (why??)
+    pub name: Option<String>,
+    pub colours: Vec<Colour>,
 }
 
 impl From<String> for Palette {