RFC: Non-filesystem interface to PROS USB serial streams

[nickmertin]

Similar to #141 but for the USB serial streams (i.e. /ser/xxxx). Essentially, we should provide a non-filesystem API for accessing a resource which is fundamentally filesystem-based, and then re-implement the filesystem one in terms of it to ensure consistency of behaviour. This would facilitate expanding the use of the terminal connection for more advanced use-cases, i.e. what this issue is trying to do for the CLI side.

Further ideas would be to extend the terminal protocol to allow for block transfers, for example. Furthermore, the terminal protocol should be properly documented.