Add commands for controling IAP mode

[21km43]

I read this issue and implemented commands which controls IAP mode.

Commands:
  iap          Enter or quit IAP mode

Example:

> wlink iap
[WARN] This is an experimental feature, better use the WCH-LinkUtility!
Please choose one mode to switch, either --enter or --quit

> wlink iap --enter
<WCH-Link#0 libusb device> Bus 001 Device 031 ID 1a86:8010(USB-FS 12 Mbps) (RV mode)
[WARN] This is an experimental feature, better use the WCH-LinkUtility!
[INFO] Enter IAP mode

> wlink iap --quit
<WCH-Link#0 libusb device> Bus 001 Device 026 ID 4348:55e0(USB-FS 12 Mbps) (IAP mode)
[WARN] This is an experimental feature, better use the WCH-LinkUtility!
[INFO] Quit IAP mode

> wlink mode-switch --dap
<WCH-Link#0 libusb device> Bus 001 Device 033 ID 1a86:8010(USB-FS 12 Mbps) (RV mode)
[WARN] This is an experimental feature, better use the WCH-LinkUtility!
[INFO] Connected to WCH-Link v2.14(v34) (WCH-LinkE-CH32V305)
[INFO] Switch mode for WCH-LinkRV

> wlink iap --enter
<WCH-Link#0 libusb device> Bus 001 Device 028 ID 1a86:8012(USB-FS 12 Mbps) (DAP mode)
[WARN] This is an experimental feature, better use the WCH-LinkUtility!
[INFO] Enter IAP mode

> wlink iap --quit
<WCH-Link#0 libusb device> Bus 001 Device 026 ID 4348:55e0(USB-FS 12 Mbps) (IAP mode)
09:12:44 [WARN] This is an experimental feature, better use the WCH-LinkUtility!
09:12:44 [INFO] Quit IAP mode

[21km43]

In my environment, Zadig is needed when quit IAP mode. (windows-x86 exe)

[21km43]

In my environment, Zadig is needed when quit IAP mode. (windows-x86 exe)

Supported controling IAP mode without Zadig (x86 only)

 CH375_DRIVER = Some(
     Library::new("WCHLinkDLL.dll")
         .map_err(|_| Error::Custom("WCHLinkDLL.dll not found".to_string()))?,
 );
-let lib = CH375_DRIVER.as_ref().unwrap();
+let mut lib = CH375_DRIVER.as_ref().unwrap();
 
+ // For IAP mode, load CH375DLL.dll if USB ID is zero
+ let get_usb_id: Symbol<unsafe extern "stdcall" fn(u32) -> u32> =
+     { lib.get(b"CH375GetUsbID").unwrap() };
+ if get_usb_id(0) == 0x0000_0000 {
+     CH375_DRIVER = Some(
+         Library::new("CH375DLL.dll")
+             .map_err(|_| Error::Custom("CH375DLL.dll not found".to_string()))?,
+     );
+    lib = CH375_DRIVER.as_ref().unwrap();
+}