Move most of the root package into a subcrate. (#1445)

This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).

crates/nu-cli/src/commands/nth.rs

@@ -0,0 +1,76 @@
+use crate::commands::WholeStreamCommand;
+use crate::context::CommandRegistry;
+use crate::prelude::*;
+use nu_errors::ShellError;
+use nu_protocol::{ReturnSuccess, Signature, SyntaxShape};
+use nu_source::Tagged;
+
+#[derive(Deserialize)]
+struct NthArgs {
+    row_number: Tagged<u64>,
+    rest: Vec<Tagged<u64>>,
+}
+
+pub struct Nth;
+
+impl WholeStreamCommand for Nth {
+    fn name(&self) -> &str {
+        "nth"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("nth")
+            .required(
+                "row number",
+                SyntaxShape::Any,
+                "the number of the row to return",
+            )
+            .rest(SyntaxShape::Any, "Optionally return more rows")
+    }
+
+    fn usage(&self) -> &str {
+        "Return only the selected rows"
+    }
+
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        args.process(registry, nth)?.run()
+    }
+}
+
+fn nth(
+    NthArgs {
+        row_number,
+        rest: and_rows,
+    }: NthArgs,
+    RunnableContext { input, .. }: RunnableContext,
+) -> Result<OutputStream, ShellError> {
+    let stream = input
+        .values
+        .enumerate()
+        .map(move |(idx, item)| {
+            let row_number = vec![row_number.clone()];
+
+            let row_numbers = vec![&row_number, &and_rows]
+                .into_iter()
+                .flatten()
+                .collect::<Vec<&Tagged<u64>>>();
+
+            let mut result = VecDeque::new();
+
+            if row_numbers
+                .iter()
+                .any(|requested| requested.item == idx as u64)
+            {
+                result.push_back(ReturnSuccess::value(item));
+            }
+
+            futures::stream::iter(result)
+        })
+        .flatten();
+
+    Ok(stream.to_output_stream())
+}