Always include the string method into the output

[mgoodfellow]

As discussed in: #45

As far as I can see, there is no reason to remove this method when targeting non-netstandard outputs (net/netcore etc).

[Turnerj]

Thanks for the PR! I've added what you suggested in the issue about using AsSpan so that the code uses a single path. I ran some benchmarks to see if there was any performance loss but I can't see any. Slight change to code size but that's not a big deal.