Initial support for HID descriptors

[martinling]

This PR adds some groundwork for supporting class-specific descriptors, where the meaning of the bDescriptorType depends on the context of the relevant interface class.

This is then used to identify and decode HID descriptors attached to an interface, where the interface class provides the context:

And to recognise requests for HID report descriptors, where the class of the recipient is known from previous traffic:

Note that HID report descriptors are not yet decoded or added to the device tree, that will come later.

[miek]

This has some conflicts now that #196 is merged, could you rebase?

[martinling]

@miek Rebased now.

[miek]

Thanks!