Started re-write (once again)

Main reason for this re-write is to be compatible with zig 0.16.0

src/util/offset_file.zig

@@ -1,8 +1,8 @@
 //! A File where it's meaningful (to us) content starts at a given offset.
 
 const std = @import("std");
-const File = std.fs.File;
-const Reader = std.fs.File.Reader;
+const File = std.Io.File;
+const Reader = File.Reader;
 
 const OffsetFile = @This();
 
@@ -13,8 +13,8 @@ pub fn init(fil: File, init_offset: u64) OffsetFile {
     return .{ .fil = fil, .offset = init_offset };
 }
 
-pub fn readerAt(self: OffsetFile, offset: u64, buffer: []u8) !Reader {
-    var rdr = self.fil.reader(buffer);
+pub fn readerAt(self: OffsetFile, io: std.Io, offset: u64, buffer: []u8) !Reader {
+    var rdr = self.fil.reader(io, buffer);
     try rdr.seekTo(self.offset + offset);
     return rdr;
 }