Encoding parity out-of-band

[scd31]

I am trying to implement the following structure:

Since the parity bits are stored separately from the data, it's a bit wasteful to create a data/parity array just to immediately separate the values again. So I wrote some methods that allow encoding the parity bits without needing to append them to the existing data array.

I also made some methods const but I can take out these changes if you prefer.

Thanks!

[scd31]

Supersedes #4

[adamgreig]

I've released this as v1.1.0 along with a couple of other tidy-ups and a performance boost to the min-sum decoder.

[scd31]

Awesome - fastest turnaround time this side of the Mississippi! :D