Pipeline blocks (#1579)

* Making Commands match what UntaggedValue needs

* WIP

* WIP

* WIP

* Moved to expressions for conditions

* Add 'each' command to use command blocks

* More cleanup

* Add test for 'each'

* Instead use an expression block

crates/nu-parser/src/lib.rs

@@ -1,5 +1,4 @@
 mod files;
-pub mod hir;
 mod lite_parse;
 mod parse;
 mod shapes;
@@ -7,10 +6,6 @@ mod signature;
 
 pub use crate::files::Files;
 pub use crate::lite_parse::{lite_parse, LitePipeline};
-pub use crate::parse::{
-    classify_pipeline, garbage, ClassifiedCommand, ClassifiedPipeline, Commands, InternalCommand,
-};
+pub use crate::parse::{classify_pipeline, garbage};
 pub use crate::shapes::shapes;
-pub use crate::signature::{
-    ExternalArg, ExternalArgs, ExternalCommand, Signature, SignatureRegistry,
-};
+pub use crate::signature::{Signature, SignatureRegistry};