Looking for a Code Review of a Bioinformatics POC

[duck_tape]

Hi! I'm new to dlang but loving it so far! One of my favorite 
first things to implement in a new language is an interval 
library. In this case I want to submit to a benchmark repo: 
https://github.com/lh3/biofast

If anyone is willing to take a look and give some feedback I'd be 
very appreciative! Specifically if you have an performance 
improvement ideas: https://github.com/sstadick/dgranges/pull/1

Currently my D version is a few seconds slower than the Crystal 
version. putting it very solid in third place overall. I'm not 
really sure where it's falling behind crystal since `-release` 
removes bounds checking. I have not looked at the assembly 
between the two, but I suspect that Crystal inlines the callback 
and D does not.

I also think there is room for improvement in the IO, as I'm just 
using the defaults.