Generic ICMPv6 message (RFC 4443).
The type-specific "message body" bytes that RFC 4443 places immediately
after the checksum (e.g. identifier/sequence for Echo, mtu for
Packet Too Big) are part of body here, along with any data that
follows them — this coder does not parse per-type layouts.
type: number
ICMPv6 type field. See ICMPV6_TYPE.
code: number
ICMPv6 code field, type-specific subtype.
checksum: number
Big-endian 16-bit checksum (RFC 8200 section 8.1) computed over an IPv6 pseudo-header plus the entire ICMPv6 message with this field set to zero. Not auto-computed on encode — the pseudo-header depends on the enclosing IPv6 addresses, which this package does not model.
body: Uint8Array<ArrayBufferLike>
Remaining bytes of the message.