message: Add Default trait for Span
  Summary: Can be useful.
diff: @@ -45,7 +45,7 @@ use std::{
  };
  /// A small, `Copy`, value representing a position in a `CodeMap`'s file.
  -#[derive(Copy, Clone, Dupe, Hash, Eq, PartialEq, Ord, PartialOrd, Debug)]
  +#[derive(Copy, Clone, Dupe, Hash, Eq, PartialEq, Ord, PartialOrd, Debug, Default)]
  pub struct Pos(u32);
  impl Add<u64> for Pos {
  @@ -63,7 +63,7 @@ impl Sub<Pos> for Pos {
  }
  /// A range of text within a CodeMap.
  -#[derive(Copy, Dupe, Clone, Hash, Eq, PartialEq, Debug)]
  +#[derive(Copy, Dupe, Clone, Hash, Eq, PartialEq, Debug, Default)]
  pub struct Span {
  /// The position in the codemap representing the first byte of the span.
  low: Pos,
  