Use Option in return type for SingleRegisterAccess::read_register

[daniel5151]

#107 has introduced an in-band mechanism of signaling cases where a single register is recognized but unavailable (i.e: returning Ok(0) from the fn)

In the next breaking version of gdbstub, this should be changed to use Option instead, making it clearer that this feature exists.

[daniel5151]

On further consideration, I think I've convinced myself that using Ok(0) to report valid but unavailable registers is actually quite reasonable.

If you think about it, read_register's API is quite similar to something like std::io::Read, where you're asked to fill up a buffer, and return a usize indicating how much data was written. With this perspective, returning 0 seems totally reasonable.

So I think I'll be closing out this issue as "won't fix"