added new line to separate scapy import
limit Descriptor length field to immediate payload

The length field of a Descriptor should only cover the immediately following
descriptor, but not further following Descriptors.
For example, a Configuration Descriptor may include multiple other descriptors.
However, the length field should only cover the initial part of the
Configuration (9 bytes).
compute total_length for Configuration Descriptor
bind Descriptor to Interface Descriptor

An Interface Descriptor may be followed by further Descriptors
added class for Strings
continued on HID 1.11
continued on Config Descriptor