Fails to build with rust nightly

When building with nightly I get the following error:

```
error: intrinsic safety mismatch between list of intrinsics within the compiler and core library intrinsics for intrinsic `type_id`
--> /usr/local/cargo/git/checkouts/libafl-c33dc6f5ec2f7a70/8eab7d6/libafl/src/observers/owned.rs:19:5
|
19 | fn type_id() -> u64;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

Which does seem to be referenced in the main libafl codebase here: https://github.com/AFLplusplus/LibAFL/blob/fc23782dc396b7700ef8a70515e4570e0a3fcf16/libafl/src/observers/mod.rs#L22

## Setup:

Using the docker `rust:latest` image:

```bash
rustup toolchain install nightly
rustup default nightly
cargo install -f cargo-libafl
```