Add recursive fibonacci benchmark (#924)

This is pretty much the least efficient reasonably simple way to
implement fibonacci in Nushell, and it's a good benchmark to profile our
custom command call performance, as this is the majority of the work
being done.

I think it may actually be good to add this and the other one to `cargo
bench` so we can track it over time, but it's also useful as a script so
it can easily be profiled.

benchmarks/fibonacci-recursive.nu

@@ -0,0 +1,17 @@
+#!/usr/bin/env nu
+# This is a much less efficient, recursive version of fibonacci that can be used to test our
+# command call performance.
+
+use std bench
+
+def fib [n: int] {
+  match $n {
+    0 => 0,
+    1 => 1,
+    $n => { (fib ($n - 1)) + (fib ($n - 2)) },
+  }
+}
+
+def main [] {
+  print (bench { 0..20 | each { |n| fib $n } } | reject times)
+}