Learn unsafe Rust: Provide content for the chapter "Intrinsics"

Provide initial content for the [Intrinsics](https://github.com/google/learn_unsafe_rust/blob/main/src/core_unsafety/intrinsics.md) chapter of the Learn unsafe Rust book.

# Basis

Rust documentation on intrinsics:

https://doc.rust-lang.org/std/intrinsics/index.html


# Folks interested

@vapdrs 
@vjonaswolf 