map() and map_mut() used to return Ref<Map>, then to convert to a concrete
type you had to:
let mut perf_map: PerfMap<_> = bpf.map_mut("PERF_MAP").unwrap().try_into()?;;
But the base Map type is pretty much only useful internally. This change
makes map() and map_mut() use TryInto internally, so that now
conversions can can be done with:
let mut perf_map: PerfMap<_> = bpf.map_mut("PERF_MAP")?.unwrap();