Rust library cleanup

* Fix error code extraction from Python to Rust
* Add documentation for dealing with HCI packets

rust/README.md

@@ -69,3 +69,68 @@ To regenerate the assigned number tables based on the Python codebase:
 ```
 PYTHONPATH=.. cargo run --bin gen-assigned-numbers --features dev-tools
 ```
+
+## HCI packets
+
+Sending a command packet from a device is composed to of two major steps.
+There are more generalized ways of dealing with packets in other scenarios.
+
+### Construct the command
+Pick a command from `src/internal/hci/packets.pdl` and construct its associated "builder" struct.
+
+```rust
+// The "LE Set Scan Enable" command can be found in the Core Bluetooth Spec.
+// It can also be found in `packets.pdl` as `packet LeSetScanEnable : Command`
+fn main() {
+    let device = init_device_as_desired();
+    
+    let le_set_scan_enable_command_builder = LeSetScanEnableBuilder {
+        filter_duplicates: Enable::Disabled,
+        le_scan_enable: Enable::Enabled,
+    };
+}
+```
+
+### Send the command and interpret the event response
+Send the command from an initialized device, and then receive the response.
+
+```rust
+fn main() {
+    // ...
+    
+    // `check_result` to false to receive the event response even if the controller returns a failure code
+    let event = device.send_command(le_set_scan_enable_command_builder.into(), /*check_result*/ false);
+    // Coerce the event into the expected format. A `Command` should have an associated event response
+    // "<command name>Complete".
+    let le_set_scan_enable_complete_event: LeSetScanEnableComplete = event.try_into().unwrap();
+}
+```
+
+### Generic packet handling
+At the very least, you should expect to at least know _which_ kind of base packet you are dealing with. Base packets in 
+`packets.pdl` can be identified because they do not extend any other packet. They are easily found with the regex:
+`^packet [^:]* \{`. For Bluetooth LE (BLE) HCI, one should find some kind of header preceding the packet with the purpose of
+packet disambiguation. We do some of that disambiguation for H4 BLE packets using the `WithPacketHeader` trait at `internal/hci/mod.rs`.
+
+Say you've identified a series of bytes that are certainly an `Acl` packet. They can be parsed using the `Acl` struct.
+```rust
+fn main() {
+    let bytes = bytes_that_are_certainly_acl();
+    let acl_packet = Acl::parse(bytes).unwrap();
+}
+```
+
+Since you don't yet know what kind of `Acl` packet it is, you need to specialize it and then handle the various
+potential cases.
+```rust
+fn main() {
+    // ...
+    match acl_packet.specialize() {
+        Payload(bytes) => do_something(bytes),
+        None => do_something_else(),
+    }
+}
+```
+
+Some packets may yet further embed other packets, in which case you may need to further specialize until no more
+specialization is needed.