Add a prelude

This prelude contains the sealed public types as well as
the conversions traits.

src/lib.rs

@@ -224,6 +224,22 @@ use {
     },
 };
 
+/// A "prelude" for users of the *fixed* crate.
+///
+/// This prelude is similar to the standard library's prelude in that you'll
+/// almost always want to import its entire contents, but unlike the standard
+/// library's prelude you'll have to do so manually:
+///
+/// ```
+/// ##[allow(unused_imports)]
+/// use fixed::prelude::*;
+/// ```
+///
+/// The prelude may grow over time as additional items see ubiquitous use.
+pub mod prelude {
+    pub use crate::traits::{FromFixed, ToFixed};
+}
+
 #[macro_use]
 mod macros_from_to;
 #[macro_use]