[Pre-RFC] BTF relocations

Oh, and I don't know if this is in scope or not, but it would be great to be able to express all of the libbpf/src/bpf_core_read.h at master · libbpf/libbpf · GitHub in Rust through whatever is the most natural way. Checks for type/field existence and enum value retrieval is critical for some real-world applications, not just relocatable field accesses.