disk based btree

fusions & memfiles seem to be a nice feature to consider, currently out of my scope i'll review it as a backgroud task, meanwhile i've 'black box' wrapped some c++ code

disk based btree

The fusion one is very "`ref object` store GC'd types in it/use the existing nim allocator". For external use you will at least need your own node allocator and probably a prohibition against (or at least very special handling of) GC'd types like `string`. [adix](https://github.com/c-blake/adix

disk based btree

Fusion has BTrees that should be easy to patch so that they support writing to disk.

disk based btree

haven't found a nim implementation for it, so i just ported a simple implementation: suitable for a fixed environment, it that just fits my needs of a 1e8 key array[10,char] file based index btree limitations: 1. fixed key & data type