Reduce memory usage

[urbanslug]

Expects sorted VCF

1. Replace Vec<T> with Box<T> for the value in data
   • would this avoid duplicates?
2. Read the whole VCF into a buffer and use non-owned slices to refer to the lines.
   • That'd avoid the heap overhead of allocations.
3. mmap the file and avoid duplicating the page cache in your process's memory
4. Serialize the VCF in fixed size chunks to disk
   • less memory but more time