Fix incircle issuees

[DanielVandH]

Fixes #20. I didn't change the incircle method to the one I implemented at https://github.com/JuliaGeometry/AdaptivePredicates.jl/pull/21 but I fixed the conditions for allocating (see the issue). I also fixed the inbounds issues by porting over the new versions of the arithmetic functions I implemented at AdaptivePredicates.jl. The safe_getindex is necessary unfortunately, even early into the function.

https://github.com/JuliaGeometry/AdaptivePredicates.jl/pull/21 gives some benchmarks with this approach to show that the speed is still good.