The scope of the unsafe block can be appropriately reduced

[peamaeq]

In these functions you use the unsafe keyword for some safe expressions. However, I found that only 3 functions are real unsafe operations (see the list below).

We need to mark unsafe operations more precisely using unsafe keyword. Keeping unsafe blocks small can bring many benefits. For example, when mistakes happen, we can locate any errors related to memory safety within an unsafe block. This is the balance between Safe and Unsafe Rust. The separation is designed to make using Safe Rust as ergonomic as possible, but requires extra effort and care when writing Unsafe Rust.
Real unsafe operation list:

1. the write_buf()\set_len()\move_head() function(these are unsafe functions)

@abonander
Hope this PR can help you.
Best regards.
References
https://doc.rust-lang.org/nomicon/safe-unsafe-meaning.html
https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html