Rename BTstack lib to BTstackLib (#1360)

Fixes #1356

Mac and Windows have case-insensitive filesystems, so the will find
the internal (all-lowercase) "btstack.h" and not the library's "BTstack.h",
causing compilation errors.

Rename the library and header to avoid the issue.

libraries/BTstack/examples/ANCS/ANCS.ino → libraries/BTstackLib/examples/ANCS/ANCS.ino

-#include <BTstack.h>
+#include <BTstackLib.h>
 #include <stdio.h>
 #include "ble/att_server.h"
 #include "ble/gatt_client.h"

libraries/BTstack/examples/LECentral/LECentral.ino → libraries/BTstackLib/examples/LECentral/LECentral.ino

-#include <BTstack.h>
+#include <BTstackLib.h>
 #include <SPI.h>
 
 /*

libraries/BTstack/examples/LEPeripheral/LEPeripheral.ino → libraries/BTstackLib/examples/LEPeripheral/LEPeripheral.ino

 // LE Peripheral Example - not working yet
-#include <BTstack.h>
+#include <BTstackLib.h>
 #include <SPI.h>
 
 /*

libraries/BTstack/examples/iBeacon/iBeacon.ino → libraries/BTstackLib/examples/iBeacon/iBeacon.ino

-#include <BTstack.h>
+#include <BTstackLib.h>
 #include <stdio.h>
 #include <SPI.h>
 

libraries/BTstack/examples/iBeaconScanner/iBeaconScanner.ino → libraries/BTstackLib/examples/iBeaconScanner/iBeaconScanner.ino

-
-#include <BTstack.h>
+#include <BTstackLib.h>
 #include <SPI.h>
 
 /* EXAMPLE_START(iBeaconScanner): iBeacon Scanner

libraries/BTstack/src/BTstack.cpp → libraries/BTstackLib/src/BTstackLib.cpp

@@ -13,7 +13,7 @@
 //#include <Reset.h>  // also provides NVIC_SystemReset
 #endif
 
-#include "BTstack.h"
+#include "BTstackLib.h"
 
 #include "btstack_memory.h"
 #include "hal_tick.h"