std::vec: add the clear method to the Vec[T]

std/vec/vec.jule

@@ -108,6 +108,12 @@ impl Vec {
         }
     }
 
+    // Removes all elements.
+    // Does not deallocates buffer, keeps capacity.
+    pub fn clear(mut self) {
+        self.mem.buff.len = 0
+    }
+
     // Push item to end of heap.
     pub fn push(mut self, mut item: T) {
         if self.len() >= self.cap() {