message: sparse: add better Debug impl
  The default impl shows a bit too many details that aren't usually what
  you want to see when debugging.
diff: +use std::fmt;
  use std::ops::Deref;
  use std::slice;
  @@ -11,7 +12,7 @@ use std::slice;
  /// Note though that we don't actually use uninitialized memory. We generally
  /// reuse allocations, so the initial allocation cost is bareable. However,
  /// its other properties listed above are extremely useful.
  -#[derive(Clone, Debug)]
  +#[derive(Clone)]
  pub struct SparseSet {
  /// Dense contains the instruction pointers in the order in which they
  /// were inserted.
  @@ -60,6 +61,12 @@ impl SparseSet {
  }
  }
  +impl fmt::Debug for SparseSet {
  + fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  + write!(f, "SparseSet({:?})", self.dense)
  + }
  +}
  +
  impl Deref for SparseSet {
  type Target = [usize];
  