Give Speed enum members the values of the various LIBUSB_SPEED_ constants

[raphaelcohn]

Give Speed enum members the values of the various LIBUSB_SPEED_ constants and the same #[repr()].

[raphaelcohn]

Likewise do this for other enums, eg Direction, Recipient, RequestType, etc.

[raphaelcohn]

If this is done the match clauses in, say request_type can be replaced with more efficient as.