`RefMutContainer` is unsound

Since I was made aware of an unsoundness in this code base today (see #1485), I decided to have a bit of a look around.

I found that `RefMutContainer` erases lifetimes, resulting in more *unsafe* code.
Source location:
https://github.com/huggingface/tokenizers/blob/14a07b06e4a8bd8f80d884419ae4630f5a3d8098/bindings/python/src/utils/mod.rs#L44-L66

Note that I didn't check if there is actually any unsoundness here. However, since the safety here can't be locally guaranteed, the methods should at least be marked as `unsafe`, but even better, fixed to be correct.

Here is an example code that shows undefined behavior when checked with Miri:
```rust
let container = {
let mut content = String::from("foo");
RefMutContainer::new(&mut content)
};

container.map(|text| {
// Triggers UB
println!("{text}");
});
```

Link to playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=fede2ac43a1fdb8b5e499b505e9ecdea

Error produced by miri:
```txt
error: Undefined Behavior: out-of-bounds pointer use: alloc1256 has been freed, so this pointer is dangling
--> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/raw.rs:138:9
|
138 | &*ptr::slice_from_raw_parts(data, len)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: alloc1256 has been freed, so this pointer is dangling
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
help: alloc1256 was allocated here:
--> src/main.rs:29:20
|
29 | let mut text = String::from("foo");
| ^^^^^^^^^^^^^^^^^^^
help: alloc1256 was deallocated here:
--> src/main.rs:31:5
|
31 | drop(text);
| ^^^^^^^^^^
= note: BACKTRACE (of the first span):
= note: inside `std::slice::from_raw_parts::<'_, u8>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/raw.rs:138:9: 138:47
= note: inside ` as std::ops::Deref>::deref` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:2831:18: 2831:64
= note: inside `::deref` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/string.rs:2487:43: 2487:52
= note: inside `::fmt` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/string.rs:2376:28: 2376:34
= note: inside `<&std::string::String as std::fmt::Display>::fmt` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/mod.rs:2377:62: 2377:82
= note: inside `core::fmt::rt::Argument::<'_>::fmt` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/rt.rs:173:76: 173:95
= note: inside `std::fmt::write` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/mod.rs:1178:21: 1178:44
= note: inside ` as std::io::Write>::write_fmt` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/io/mod.rs:1823:15: 1823:43
= note: inside `<&std::io::Stdout as std::io::Write>::write_fmt` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/io/stdio.rs:786:9: 786:36
= note: inside `::write_fmt` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/io/stdio.rs:760:9: 760:33
= note: inside `std::io::stdio::print_to::` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/io/stdio.rs:1116:21: 1116:47
= note: inside `std::io::_print` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/io/stdio.rs:1226:5: 1226:37
note: inside closure
--> src/main.rs:35:7
|
35 | println!("{text}");
| ^^^^^^^^^^^^^^^^^^
note: inside `RefMutContainer::::map::<{closure@src/main.rs:33:19: 33:25}, ()>`
--> src/main.rs:18:14
|
18 | Some(f(unsafe { ptr.as_ref().unwrap() }))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `main`
--> src/main.rs:33:5
|
33 | / container.map(|text| {
34 | | // Triggers UB
35 | | println!("{text}");
36 | | });
| |______^
= note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
```

Edit: Some updates for clarity