This commit makes it so `!Unpin` types can be passed into `io::copy`.

Signed-off-by: John Nunley <[email protected]>

Signed-off-by: John Nunley <[email protected]>

This respects rust-version field in Cargo.toml, so it removes the need
to manage MSRV in both the CI file and Cargo.toml.

Since #77, this crate works
on 1.60.

```
warning: redundant guard
    --> src/io.rs:2701:28
     |
2701 |                 Ok(buf) if buf.is_empty() => {
     |                            ^^^^^^^^^^^^^^
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_guards
     = note: `#[warn(clippy::redundant_guards)]` on by default
help: try
     |
2701 -                 Ok(buf) if buf.is_empty() => {
2701                   Ok([]) => {
     |
```