Move the USB-HID authenticator transport implementations into a subfolder/module #124
Description
Since I intend (as part of #123 and others) to add additional backends beyond the existing USB HID mechanism, I want to fairly significantly change the layout of this, though I want to mostly settle the rearchitecture work first.
Firstly, src/<platform> is insufficient. Everything currently there is really a USB-HID implementation, so the most straightforward change is to do basically:
mkdir src/transports/usb-hid/; git mv src/{macos,netbsd,freebsd,linux,openbsd,stub,windows} src/transports/usb-hid/
and of course update all module names.
Also, src/manager.rs should move into src/transports/usb-hid
I am also wondering at moving src/u2f* and the relevant hid and consts into their own directory, but that might just have to come as a later refactor.