Encoding length

okay this follows on from #6,

adds an encodingLength function that returns the length a particular int _will be_ encoded as,
useful if you need to know in order to allocate buffers or such like.
Used in https://github.com/dominictarr/varstruct
(actually, that uses `dynamicLength` - but I agree with you `encodingLength` is a better name,
I'll change it after it's merged.)