std.file, std.stdio(File), std.stream(File:Stream)

[ref2401]

general question:
-std.file,
-std.stdio.File,
-std.stream.File

which of the existing approaches i have to choose when working 
with files? what shall i look at?

for example, i have a binary file. i want to read the first byte, 
process it and then read the following N bytes (N calculation 
based on the first byte value), process this chunk and then read 
the file to the end.