feat(solutions): add fibonacci/rust/function

challenges/fibonacci/solutions/rust/function/src/main.rs

@@ -0,0 +1,20 @@
+use std::io;
+
+pub fn fibonacci(number: u32) -> u32 {
+    if number == 0 || number == 1 {
+        return number;
+    }
+    return fibonacci(number - 1) + fibonacci(number - 2);
+}
+
+fn main() {
+    let mut number = String::new();
+    io::stdin()
+        .read_line(&mut number)
+        .expect("Failed to read `stdin` line.");
+    let number: u32 = number
+        .trim()
+        .parse()
+        .expect("Failed to convert `number` as an `u32`.");
+    println!("{}", fibonacci(number));
+}