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-cli/tests/commands/mod.rs

@@ -4,6 +4,7 @@ mod cd;
 mod compact;
 mod cp;
 mod default;
+mod each;
 mod edit;
 mod enter;
 mod first;