Using traits to share implementations

[FIXME] Elaborate.

[FIXME] We probably want to discourage this, at least when used in a way that is publicly exposed.

Traits that provide default implmentations for function can provide code reuse across types. For example, a print method can be defined across multiple types as follows:

trait Printable {
    // Default method implementation
    fn print(&self) { println!("{:?}", *self) }

impl Printable for int {}

impl Printable for String {
    fn print(&self) { println!("{}", *self) }

impl Printable for bool {}

impl Printable for f32 {}

This allows the implementation of print to be shared across types, yet overridden where needed, as seen in the impl for String.