Add export alias and export extern (#4878)

* export alias

* export extern

crates/nu-protocol/src/engine/engine_state.rs

@@ -770,6 +770,19 @@ impl<'a> StateWorkingSet<'a> {
         }
     }
 
+    pub fn use_aliases(&mut self, aliases: Vec<(Vec<u8>, AliasId)>) {
+        let scope_frame = self
+            .delta
+            .scope
+            .last_mut()
+            .expect("internal error: missing required scope frame");
+
+        for (name, alias_id) in aliases {
+            scope_frame.aliases.insert(name, alias_id);
+            scope_frame.visibility.use_decl_id(&alias_id);
+        }
+    }
+
     pub fn add_predecl(&mut self, decl: Box<dyn Command>) -> Option<DeclId> {
         let name = decl.name().as_bytes().to_vec();
 

crates/nu-protocol/src/exportable.rs

@@ -1,6 +1,7 @@
-use crate::{BlockId, DeclId};
+use crate::{AliasId, BlockId, DeclId};
 
 pub enum Exportable {
     Decl(DeclId),
+    Alias(AliasId),
     EnvVar(BlockId),
 }

crates/nu-protocol/src/overlay.rs

@@ -116,6 +116,18 @@ impl Overlay {
             .collect()
     }
 
+    pub fn aliases_with_head(&self, head: &[u8]) -> Vec<(Vec<u8>, AliasId)> {
+        self.aliases
+            .iter()
+            .map(|(name, id)| {
+                let mut new_name = head.to_vec();
+                new_name.push(b' ');
+                new_name.extend(name);
+                (new_name, *id)
+            })
+            .collect()
+    }
+
     pub fn alias_names_with_head(&self, head: &[u8]) -> Vec<Vec<u8>> {
         self.aliases
             .keys()