BTF relocations

The RFC proposes the `btf_relocations` feature gate, which provides a
`#[repr(Btf)]` representation and `core::btf` field-info macros that
emit BTF (BPF Type Format)[0] CO-RE (Compile Once, Run Everywhere)[1]
relocations.

It also documents the relationship to `offset_of!`, ordinary field
projection, and LLVM BPF lowering.

This feature was originally proposed as a pre-RFC[2].

[0] https://docs.kernel.org/bpf/btf.html
[1] https://nakryiko.com/posts/bpf-portability-and-co-re/
[2] https://internals.rust-lang.org/t/pre-rfc-btf-relocations/24161/25

Author: vadorovsky