Adding `read_with_seq` method

[extremeandy]

Would you be open to an MR adding a new method read_with_seq which returns the current value of seq (i.e. seq1 which was loaded and compared with seq2)?

I'd like to compose a channel around SeqLock which would behave like tokio::watch except with lock-free writes. It would be useful to be able to use the seq number when reading to be able to determine whether or not the data had been written since last read.

[corvusrabus]

I think if you'd like to use this with tokio you'd also need a tokio feature flag that replaces the thread::yield_now(); with tokio::yield_now()

[Amanieu]

I think there is value in a read_with_seq API to detect whether a change happened. However it would be best if the sequence number was wrapped in an opaque type that just provided Eq. This retains flexibility to change the integer type used for the sequence number in the future.