wire/dhcp: remove option enum, letting higher layers specify kind+data.

This saves ~1.5kb of code size.

Options like that generate a bit of code size bloat due to having many code paths that
the compiler isn't able to optimize out. Also they don't scale well, adding every single
option will make the bloat worse.

Author: Dirbaio

src/socket/dhcpv4.rs

@@ -3,12 +3,12 @@ use core::task::Waker;
 
 use crate::iface::Context;
 use crate::time::{Duration, Instant};
-use crate::wire::dhcpv4::{field as dhcpv4_field, DhcpOptionsBuffer};
-use crate::wire::HardwareAddress;
+use crate::wire::dhcpv4::field as dhcpv4_field;
 use crate::wire::{
     DhcpMessageType, DhcpPacket, DhcpRepr, IpAddress, IpProtocol, Ipv4Address, Ipv4Cidr, Ipv4Repr,
     UdpRepr, DHCP_CLIENT_PORT, DHCP_MAX_DNS_SERVER_COUNT, DHCP_SERVER_PORT, UDP_HEADER_LEN,
 };
+use crate::wire::{DhcpOption, HardwareAddress};
 
 #[cfg(feature = "async")]
 use super::WakerRegistration;
@@ -150,7 +150,7 @@ pub struct Socket<'a> {
 
     /// A buffer contains options additional to be added to outgoing DHCP
     /// packets.
-    outgoing_options: Option<DhcpOptionsBuffer<&'a [u8]>>,
+    outgoing_options: &'a [DhcpOption<'a>],
     /// A buffer containing all requested parameters.
     parameter_request_list: Option<&'a [u8]>,
 
@@ -180,7 +180,7 @@ impl<'a> Socket<'a> {
             max_lease_duration: None,
             retry_config: RetryConfig::default(),
             ignore_naks: false,
-            outgoing_options: None,
+            outgoing_options: &[],
             parameter_request_list: None,
             receive_packet_buffer: None,
             #[cfg(feature = "async")]
@@ -193,9 +193,9 @@ impl<'a> Socket<'a> {
         self.retry_config = config;
     }
 
-    /// Set the outgoing options buffer.
-    pub fn set_outgoing_options_buffer(&mut self, options_buffer: DhcpOptionsBuffer<&'a [u8]>) {
-        self.outgoing_options = Some(options_buffer);
+    /// Set the outgoing options.
+    pub fn set_outgoing_options(&mut self, options: &'a [DhcpOption<'a>]) {
+        self.outgoing_options = options;
     }
 
     /// Set the buffer into which incoming DHCP packets are copied into.
@@ -841,7 +841,7 @@ mod test {
         dns_servers: None,
         max_size: None,
         lease_duration: None,
-        additional_options: None,
+        additional_options: &[],
     };
 
     const DHCP_DISCOVER: DhcpRepr = DhcpRepr {