message: Better capacity display for Vec
  Summary:
  Add a layer "capacity" below data and "unused_capacity".
  So if unused capacity is zero, make it more clear: all data lies within capacity.
diff: use std::mem;
  use crate::allocative_trait::Allocative;
  +use crate::impls::common::CAPACITY_NAME;
  use crate::impls::common::UNUSED_CAPACITY_NAME;
  use crate::key::Key;
  @@ -173,11 +174,13 @@ impl<'a> Visitor<'a> {
  'a: 'b,
  T: Allocative,
  {
  - self.visit_slice(data);
  - self.visit_simple(
  + let mut visitor = self.enter(CAPACITY_NAME, mem::size_of::<T>() * capacity);
  + visitor.visit_slice(data);
  + visitor.visit_simple(
  UNUSED_CAPACITY_NAME,
  mem::size_of::<T>() * capacity.wrapping_sub(data.len()),
  );
  + visitor.exit();
  }
  fn exit_impl(&mut self) {
  