Unsafe pointer mutation

[tobywf]

For the std::io::Write implementation on TTYPort, a *const ptr is cast to *mut:

https://github.com/dcuddeback/serial-rs/blob/cb28b1439a5653f1777ad11d85d37f2c8074259b/serial-unix/src/tty.rs#L149

This is unnecessary, as libc::write takes buf: *const c_void, so buf.as_ptr() as *const c_void would work fine.

For the std::io::Read implementation on TTYPort, similar but worse, as the pointer is mutated:

https://github.com/dcuddeback/serial-rs/blob/cb28b1439a5653f1777ad11d85d37f2c8074259b/serial-unix/src/tty.rs#L132

In this case, the solution is buf.as_mut_ptr() as *mut c_void. The documentation on slice.as_ptr():

The caller must also ensure that the memory the pointer (non-transitively) points to is never written to (except inside an UnsafeCell) using this pointer or any pointer derived from it. If you need to mutate the contents of the slice, use as_mut_ptr.

I haven't checked any other code, so there may be more.