Resolve std_instead_of_alloc clippy restriction

    warning: used import from `std` instead of `alloc`
       --> src/unique_ptr.rs:201:41
        |
    201 |     fn read_to_end(&mut self, buf: &mut std::vec::Vec<u8>) -> io::Result<usize> {
        |                                         ^^^ help: consider importing the item from `alloc`: `alloc`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_alloc
    note: the lint level is defined here
       --> src/lib.rs:377:5
        |
    377 |     clippy::std_instead_of_alloc,
        |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    warning: used import from `std` instead of `alloc`
       --> src/unique_ptr.rs:206:44
        |
    206 |     fn read_to_string(&mut self, buf: &mut std::string::String) -> io::Result<usize> {
        |                                            ^^^ help: consider importing the item from `alloc`: `alloc`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_alloc

src/unique_ptr.rs

@@ -198,12 +198,12 @@ where
     }
 
     #[inline]
-    fn read_to_end(&mut self, buf: &mut std::vec::Vec<u8>) -> io::Result<usize> {
+    fn read_to_end(&mut self, buf: &mut alloc::vec::Vec<u8>) -> io::Result<usize> {
         self.pin_mut().read_to_end(buf)
     }
 
     #[inline]
-    fn read_to_string(&mut self, buf: &mut std::string::String) -> io::Result<usize> {
+    fn read_to_string(&mut self, buf: &mut alloc::string::String) -> io::Result<usize> {
         self.pin_mut().read_to_string(buf)
     }