Be more Rusty

[gatesn]

Use traits in a more canonical Rust-like way!

End State:

/// Internal API for an Array
pub trait ArrayImpl {
  fn len(&self) -> usize;
  fn children(&self) -> &[ArrayRef];
  ...
}

/// Public API for an array
pub struct Array(Arc<dyn ArrayImpl>);
impl Array {
  
}

pub trait ArrayCodecImpl {
  fn id(&self) -> ArrayId;
  fn deserialize(&self, ...) -> ...
}
pub struct ArrayCodec(Arc<dyn ArrayCodecImpl>);

• Remove all instances of &dyn Array for &ArrayRef.
• Create ArrayImpl trait.
• Create `struct ArrayRef(Arc);
• Rename ArrayRef -> Array... eventually
• Repeat for Expr, Layout, ExtDType.