Add environment variable support for modules (#331)

* Add 'expor env' dummy command

* (WIP) Abstract away module exportables as Overlay

* Switch to Overlays for use/hide

Works for decls only right now.

* Fix passing import patterns of hide to eval

* Simplify use/hide of decls

* Add ImportPattern as Expr; Add use env eval

Still no parsing of "export env" so I can't test it yet.

* Refactor export parsing; Add InternalError

* Add env var export and activation; Misc changes

Now it is possible to `use` env var that was exported from a module.

This commit also adds some new errors and other small changes.

* Add env var hiding

* Fix eval not recognizing hidden decls

Without this change, calling `hide foo`, the evaluator does not know
whether a custom command named "foo" was hidden during parsing,
therefore, it is not possible to reliably throw an error about the "foo"
name not found.

* Add use/hide/export env var tests; Cleanup; Notes

* Ignore hide env related tests for now

* Fix main branch merge mess

* Fixed multi-word export def

* Fix hiding tests on Windows

* Remove env var hiding for now

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

@@ -131,6 +131,7 @@ impl Expression {
                 }
                 false
             }
+            Expr::ImportPattern(_) => false,
             Expr::Filepath(_) => false,
             Expr::Float(_) => false,
             Expr::FullCellPath(full_cell_path) => {
@@ -282,6 +283,7 @@ impl Expression {
                     .head
                     .replace_in_variable(working_set, new_var_id);
             }
+            Expr::ImportPattern(_) => {}
             Expr::Garbage => {}
             Expr::GlobPattern(_) => {}
             Expr::Int(_) => {}