Improvements to encoding of natural numbers

[RichardWarburton]

I've been switching Artio codecs over to using Agrona's Ascii number encoding and decoding. I've noticed a couple of missing features that are in Artio that aren't there in the Agrona equivalents relating to natural numbers:

1. Artio has a putNatural() method that puts a value within a certain size range. So it zero pads the beginning of the size range, and also throws an exception if the value goes out of the range. zero padding can be useful if you've pre-allocated a space within a buffer that you want to encode a number into.

2. Artio has a putNaturalFromEnd() method that instead of taking a start index, takes an end index and puts the number before that point. This gives you more encoding options, especially if you've got one final field that you want to fill in at the start of an object, eg a length field, that you don't know the encoded size of ahead of time.

Are you interested in PRs for either of these methods or do you consider them too specialized for Agrona?

[mjpt777]

Sure @RichardWarburton send a PR.