Editing an open file is often janky

There's code that's supposed to stitch up the file in memory when an open file is modified (https://github.com/chalcolith/eohippus/blob/main/eohippus/server/src_file_info.pony#L54), but it seems quite janky.  Investigate to make sure the file is getting stitched together correctly, and that the parser can correctly re-parse.