chore: bump MSRV to 1.57

1.56 would be enough for miniz_oxide 0.8 and edition 2021,
but bytes may bump it to 1.57 soon
tokio-rs/bytes#613 (comment)

.github/workflows/ci.yml

@@ -63,8 +63,8 @@ jobs:
           - name: linux / nightly -Z minimal-versions
             rust: nightly
             args: -Z minimal-versions
-          - name: linux / 1.40.0
-            rust: 1.40.0
+          - name: linux / 1.57.0
+            rust: 1.57.0
 
     steps:
       - name: Checkout

README.md

@@ -3,7 +3,7 @@
 [![crates.io](https://img.shields.io/crates/v/img-parts.svg)](https://crates.io/crates/img-parts)
 [![Documentation](https://docs.rs/img-parts/badge.svg)](https://docs.rs/img-parts)
 [![dependency status](https://deps.rs/crate/img-parts/0.3.0/status.svg)](https://deps.rs/crate/img-parts/0.3.0)
-[![Rustc Version 1.40.0+](https://img.shields.io/badge/rustc-1.40.0+-lightgray.svg)](https://blog.rust-lang.org/2019/12/19/Rust-1.40.0.html)
+[![Rustc Version 1.57.0+](https://img.shields.io/badge/rustc-1.57.0+-lightgray.svg)](https://blog.rust-lang.org/2021/12/02/Rust-1.57.0.html)
 [![CI](https://github.com/paolobarbolini/img-parts/workflows/CI/badge.svg)](https://github.com/paolobarbolini/img-parts/actions?query=workflow%3ACI)
 
 The `img-parts` crate provides a low level API for reading and