Separate Overlay into its own thing (#344)

It's no longer attached to a Block. Makes access to overlays more
streamlined by removing this one indirection. Also makes it easier to
create standalone overlays without a block which might come in handy.

crates/nu-protocol/src/ast/block.rs

@@ -1,6 +1,6 @@
 use std::ops::{Index, IndexMut};
 
-use crate::{Overlay, Signature, VarId};
+use crate::{Signature, VarId};
 
 use super::Statement;
 
@@ -8,7 +8,6 @@ use super::Statement;
 pub struct Block {
     pub signature: Box<Signature>,
     pub stmts: Vec<Statement>,
-    pub overlay: Overlay,
     pub captures: Vec<VarId>,
 }
 
@@ -47,19 +46,9 @@ impl Block {
         Self {
             signature: Box::new(Signature::new("")),
             stmts: vec![],
-            overlay: Overlay::new(),
             captures: vec![],
         }
     }
-
-    pub fn with_overlay(self, overlay: Overlay) -> Self {
-        Self {
-            signature: self.signature,
-            stmts: self.stmts,
-            overlay,
-            captures: self.captures,
-        }
-    }
 }
 
 impl<T> From<T> for Block
@@ -70,7 +59,6 @@ where
         Self {
             signature: Box::new(Signature::new("")),
             stmts: stmts.collect(),
-            overlay: Overlay::new(),
             captures: vec![],
         }
     }