net: change CNetAddr::ip to have flexible size

Before this change `CNetAddr::ip` was a fixed-size array of 16 bytes,
not being able to store larger addresses (e.g. TORv3) and encoded
smaller ones as 16-byte IPv6 addresses.

Change its type to `prevector`, so that it can hold larger addresses and
do not disguise non-IPv6 addresses as IPv6. So the IPv4 address
`1.2.3.4` is now encoded as `01020304` instead of
`00000000000000000000FFFF01020304`.

Rename `CNetAddr::ip` to `CNetAddr::m_addr` because it is not an "IP" or
"IP address" (TOR addresses are not IP addresses).

In order to preserve backward compatibility with serialization (where
e.g. `1.2.3.4` is serialized as `00000000000000000000FFFF01020304`)
introduce `CNetAddr` dedicated legacy serialize/unserialize methods.

Adjust `CSubNet` accordingly. Still use `CSubNet::netmask[]` of fixed 16
bytes, but use the first 4 for IPv4 (not the last 4). Only allow
subnetting for IPv4 and IPv6.

Co-authored-by: Carl Dong <contact@carldong.me>

src/util/strencodings.cpp

@@ -318,6 +318,18 @@ bool ParseInt64(const std::string& str, int64_t *out)
         n <= std::numeric_limits<int64_t>::max();
 }
 
+bool ParseUInt8(const std::string& str, uint8_t *out)
+{
+    uint32_t u32;
+    if (!ParseUInt32(str, &u32) || u32 > std::numeric_limits<uint8_t>::max()) {
+        return false;
+    }
+    if (out != nullptr) {
+        *out = static_cast<uint8_t>(u32);
+    }
+    return true;
+}
+
 bool ParseUInt32(const std::string& str, uint32_t *out)
 {
     if (!ParsePrechecks(str))