modification after review

Cross check the list length between `extra_lang_ids`, `manufacturer`, `product` and `serial_number` on constructing `UsbDeviceBuilder`.
Build `lang_id_bytes` with rust iterators in `descriptor_type::STRING` of `fn get_descriptor()`.
A quick impl `LangID` enum.

Author: eZioPan

src/class.rs

@@ -1,6 +1,7 @@
 use crate::bus::{InterfaceNumber, StringIndex, UsbBus};
 use crate::control;
 use crate::control_pipe::ControlPipe;
+use crate::descriptor::lang_id::LangID;
 use crate::descriptor::{BosWriter, DescriptorWriter};
 use crate::endpoint::EndpointAddress;
 use crate::{Result, UsbError};
@@ -43,7 +44,7 @@ pub trait UsbClass<B: UsbBus> {
     /// * `index` - A string index allocated earlier with
     ///   [`UsbAllocator`](crate::bus::UsbBusAllocator).
     /// * `lang_id` - The language ID for the string to retrieve.
-    fn get_string(&self, index: StringIndex, lang_id: u16) -> Option<&str> {
+    fn get_string(&self, index: StringIndex, lang_id: LangID) -> Option<&str> {
         let _ = (index, lang_id);
         None
     }

src/descriptor.rs

@@ -19,12 +19,7 @@ pub mod descriptor_type {
 }
 
 /// String descriptor language IDs.
-pub mod lang_id {
-    /// English (US)
-    ///
-    /// Recommended for use as the first language ID for compatibility.
-    pub const ENGLISH_US: u16 = 0x0409;
-}
+pub mod lang_id;
 
 /// Standard capability descriptor types
 #[allow(missing_docs)]