add compare and llwu register defs

Add Serial.clear() to Teensy 3.x

On Teensy 3.x - we have Serial1.clear, Serial2.clear...

But not Serial.clear.

Serial.clear() exists for Teensy2, so thought I would try to add it for
Teensy 3s...

Allow the teensy3 library to be used with strict C++11 conformance.

Add volume/mute feature to AudioInputUSB

Add support for controlling left/right volume separately. Fix
wTotalLength after adding volume feature.

This enables left/right balance to be implemented and is more
consistent with other devices

This reverts commit c083b394e4.

Revise definition to be consistent with other usage of byte

Change descriptor and code to handle features for USB Audio

copy_to_buffers optimize..

https://forum.pjrc.com/threads/33017-Teensy-3-1-Mouse-horizontal-scroll

assembler does not help much, i think.. ?
+ perhaps make len const