UPDATE: Adds 56 functions in the Statistical section

Uses statrs for numerical functions

REFACTOR: Put statistical functions on its own module

This might seem counter-intuitive but the wasm build after this refactor
is 1528 bytes smaller :)

base/src/test/statistical/test_fn_expon_dist.rs

@@ -0,0 +1,32 @@
+#![allow(clippy::unwrap_used)]
+
+use crate::test::util::new_empty_model;
+
+#[test]
+fn test_fn_expon_dist_smoke() {
+    let mut model = new_empty_model();
+
+    // λ = 1, x = 0.5
+    // CDF = 1 - e^-0.5 ≈ 0.393469340
+    // PDF = e^-0.5 ≈ 0.606530660
+    model._set("A1", "=EXPON.DIST(0.5, 1, TRUE)");
+    model._set("A2", "=EXPON.DIST(0.5, 1, FALSE)");
+
+    // Wrong number of args
+    model._set("A3", "=EXPON.DIST(0.5, 1)");
+    model._set("A4", "=EXPON.DIST(0.5, 1, TRUE, FALSE)");
+
+    // Domain errors
+    model._set("A5", "=EXPON.DIST(-1, 1, TRUE)"); // x < 0
+    model._set("A6", "=EXPON.DIST(0.5, 0, TRUE)"); // lambda <= 0
+
+    model.evaluate();
+
+    assert_eq!(model._get_text("A1"), *"0.39346934");
+    assert_eq!(model._get_text("A2"), *"0.60653066");
+
+    assert_eq!(model._get_text("A3"), *"#ERROR!");
+    assert_eq!(model._get_text("A4"), *"#ERROR!");
+    assert_eq!(model._get_text("A5"), *"#NUM!");
+    assert_eq!(model._get_text("A6"), *"#NUM!");
+}