Known nat width

[phadej]

Builds on my other PR. The drawback of this approach is that pretty-compact would work only with GHC>=7.8.

Another option is to change newtype Doc a = MkDoc (Reader Int [(M a, L a)]).

[phadej]

I'll check the Reader approach too when the #4 is merged. It's easier to check the performance affects when we have some benchmarks in place.

[jyp]

I'm wondering if using a "KnownNat" isn't overkill. We could be using a simple parameter to the render function, like every pretty printing library does.

[phadej]

I'll do a parameter version shortly.

[phadej]

@jyp check my other PR #6