Add the bgp-rpki crate: route origin validation and an RTR client

The first part of RPKI origin validation (RFC 6811): the data and how it
gets here. Nothing uses it yet.

VrpTable holds the validated ROA payloads and says whether a route is
valid, invalid or not found. It is a binary trie in one allocation with
a short cut past the first sixteen levels; a lookup against 600,000
entries takes about 40 ns for random addresses. A table is built once
and not changed: when the data changes another takes its place.

rtr is the RPKI-to-Router protocol's PDUs (RFC 8210, and version 0 of
RFC 6810), as bytes and back, without I/O. Lengths are checked from the
header, before the rest of a PDU is waited for.

The client keeps the table in step with a validator's cache. It asks
for everything once and then for changes, when the cache notifies and
at the refresh interval. An answer takes effect only when it is complete
and consistent. Each update says which VRPs came or went, so that only
the routes they cover need looking at again. A cache that fails is
retried and the next one tried; the data is used meanwhile, and dropped
if no cache confirms it within the expire interval. A cache that speaks
only version 0 is spoken to in version 0.

Tests: the cases of RFC 6811, a property test of the trie against the
plain definition, round-trip and hostile-input properties for the
codec, a fuzz target, and the client against a scripted cache.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

fuzz/fuzz_targets/rtr.rs

@@ -0,0 +1,27 @@
+//! Bytes from an RPKI cache: a stream of RTR PDUs. A cache is trusted for
+//! its data, not to be free of bugs or to be the cache we meant to reach.
+#![no_main]
+
+use bgp_rpki::rtr::Pdu;
+use bytes::BytesMut;
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    let mut rest = data;
+    // As the client reads: one PDU after another until one is incomplete
+    // or wrong.
+    while let Ok(Some((version, pdu, used))) = Pdu::decode(rest) {
+        assert!(used >= 8 && used <= rest.len());
+        // What was read can be shown and written.
+        let _ = format!("{pdu:?}");
+        let mut out = BytesMut::new();
+        pdu.encode(version, &mut out);
+        // And what is written reads back as itself. A router key is read
+        // only to be skipped, and an error report is written without the
+        // PDU it complains about.
+        if !matches!(pdu, Pdu::RouterKey | Pdu::ErrorReport { .. }) {
+            assert_eq!(Pdu::decode(&out), Ok(Some((version, pdu, out.len()))));
+        }
+        rest = &rest[used..];
+    }
+});