Externals now spawn independently. (#1230)

This commit changes the way we shell out externals when using the `"$it"` argument. Also pipes per row to an external's stdin if no `"$it"` argument is present for external commands. 

Further separation of logic (preparing the external's command arguments, getting the data for piping, emitting values, spawning processes) will give us a better idea for lower level details regarding external commands until we can find the right abstractions for making them more generic and unify within the pipeline calling logic of Nu internal's and external's.

crates/nu-parser/src/commands/classified/external.rs

@@ -6,6 +6,16 @@ pub struct ExternalArg {
     pub tag: Tag,
 }
 
+impl ExternalArg {
+    pub fn has(&self, name: &str) -> bool {
+        self.arg == name
+    }
+
+    pub fn is_it(&self) -> bool {
+        self.has("$it")
+    }
+}
+
 impl std::ops::Deref for ExternalArg {
     type Target = str;
 
@@ -42,6 +52,12 @@ pub struct ExternalCommand {
     pub args: ExternalArgs,
 }
 
+impl ExternalCommand {
+    pub fn has_it_argument(&self) -> bool {
+        self.args.iter().any(|arg| arg.has("$it"))
+    }
+}
+
 impl PrettyDebug for ExternalCommand {
     fn pretty(&self) -> DebugDocBuilder {
         b::typed(