feat: add pi

__std/pi/src/main.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let mut pi = 1f64;
+    for i in 1..1000000000u64 {
+        pi += if i % 2 == 1 { -1f64 } else { 1f64 } / (1f64 + i as f64 * 2f64);
+    }
+    println!("pi = {}", pi * 4f64);
+}