Hiding of environment variables (#362)

* Remember environment variables from previous scope

* Re-introduce env var hiding

Right now, hiding decls is broken

* Re-introduce hidden field of import patterns

All tests pass now.

* Remove/Address tests TODOs

* Fix test typo; Report hiding error

* Add a few more tests

* Fix wrong expected test result

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

@@ -1,4 +1,5 @@
 use crate::{span, Span};
+use std::collections::HashSet;
 
 #[derive(Debug, Clone)]
 pub enum ImportPatternMember {
@@ -17,6 +18,9 @@ pub struct ImportPatternHead {
 pub struct ImportPattern {
     pub head: ImportPatternHead,
     pub members: Vec<ImportPatternMember>,
+    // communicate to eval which decls/aliases were hidden during `parse_hide()` so it does not
+    // interpret these as env var names:
+    pub hidden: HashSet<Vec<u8>>,
 }
 
 impl ImportPattern {
@@ -37,4 +41,12 @@ impl ImportPattern {
 
         span(&spans)
     }
+
+    pub fn with_hidden(self, hidden: HashSet<Vec<u8>>) -> Self {
+        ImportPattern {
+            head: self.head,
+            members: self.members,
+            hidden,
+        }
+    }
 }