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-protocol/src/syntax_shape.rs

@@ -30,6 +30,8 @@ pub enum SyntaxShape {
     Unit,
     /// An operator
     Operator,
+    /// A condition, eg `foo > 1`
+    Condition,
 }
 
 impl PrettyDebug for SyntaxShape {
@@ -49,6 +51,7 @@ impl PrettyDebug for SyntaxShape {
             SyntaxShape::Table => "table",
             SyntaxShape::Unit => "unit",
             SyntaxShape::Operator => "operator",
+            SyntaxShape::Condition => "condition",
         })
     }
 }