Bump MSRV to 1.57 (#788)

tests/test_buf_mut.rs

@@ -4,7 +4,6 @@ use bytes::buf::UninitSlice;
 use bytes::{BufMut, BytesMut};
 use core::fmt::Write;
 use core::mem::MaybeUninit;
-use core::usize;
 
 #[test]
 fn test_vec_as_mut_buf() {

tests/test_bytes.rs

@@ -5,7 +5,6 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;
 
 use std::panic::{self, AssertUnwindSafe};
-use std::usize;
 
 const LONG: &[u8] = b"mary had a little lamb, little lamb, little lamb";
 const SHORT: &[u8] = b"hello world";
@@ -82,7 +81,6 @@ fn fmt() {
 #[test]
 fn fmt_write() {
     use std::fmt::Write;
-    use std::iter::FromIterator;
     let s = String::from_iter((0..10).map(|_| "abcdefg"));
 
     let mut a = BytesMut::with_capacity(64);