Fix debug assertions in Rust 1.78+

[adierking]

Rust 1.78 introduced unsafe precondition checks when debug assertions are enabled (rust-lang/rust#120594), and they are causing this crate to panic in debug builds:

thread 'tests::drain_filter_complex' panicked at library/core/src/panicking.rs:156:5:
unsafe precondition(s) violated: slice::get_unchecked_mut requires that the index is within the slice

This PR fixes two categories of errors:

1. get_unchecked_mut() will panic if you try to get a pointer past the last element of the slice. The fix here is to ensure that move_tail_unchecked() always gets called first with a non-zero value.
2. slice::from_raw_parts() will panic if the slice is unaligned. I'm not sure why as_slices() ever needed to use this, and I don't see any issue with just using an empty slice literal (&[]).

With this I'm able to pass the full test suite on both Rust 1.78 and nightly.