add solutions for basic-types, fix errors

solutions/basic-types/functions.md

@@ -0,0 +1,55 @@
+1.
+```rust
+fn main() {
+    // don't modify the following two lines!
+    let (x, y) = (1, 2);
+    let s = sum(x, y);
+
+    assert_eq!(s, 3);
+}
+
+fn sum(x: i32, y: i32) -> i32 {
+    x + y
+}
+```
+
+2.
+```rust
+fn main() {
+    print();
+}
+
+// replace i32 with another type
+fn print() -> () {
+    println!("hello,world");
+}
+```
+
+3.
+```rust
+fn main() {
+    never_return();
+}
+
+fn never_return() -> ! {
+    // implement this function, don't modify fn signatures
+    panic!("I return nothing!")
+}
+```
+
+```rust
+fn main() {
+    never_return();
+}
+
+use std::thread;
+use std::time;
+fn never_return() -> ! {
+    // implement this function, don't modify fn signatures
+    loop {
+        println!("I return nothing");
+        // sleeping for 1 second to avoid exhausting the cpu resoucre
+        thread::sleep(time::Duration::from_secs(1))
+    }
+}
+```