Move teensy_guess_board into teensy.c

Add platform-specific hid_read_descriptor to get relevant
information (usage, usage_page) for detection purposes.
Fix broken list code on Windows
Move teensy_list into teensy.c

teensy_list now calls _teensy_list_class_devices to list serial
and HID devices.
Move some platform-specific code into <platform>/util.c

Besides, fix millis() API and return uint64_t. Add now() for
time in seconds since epoch.
Cosmetic changes
Move teensy_probe and teensy_close into teensy.c

Platform-dependent parts are extracted into _teensy_open_device
and _teensy_close_device.