Issue when using explicit i64 on 32 bit machines #15

Hi, I've encountered the following issue: i_triangle can map polygons to i64 when specified, but i_key_sort::SortKey for i64, still computes key distances as usize.

In a 32bit environment, such as wasm32, that will truncate the 64-bit distance; the bin layout is then sized from one truncated value while indexing uses another, producing a bin index beyond its 256-entry mapper.