std.io: move getStdIn, getStdOut, getStdErr functions to fs.File

preparing to rearrange std.io namespace into an interface

how to upgrade:

std.io.getStdIn() -> std.fs.File.stdin()
std.io.getStdOut() -> std.fs.File.stdout()
std.io.getStdErr() -> std.fs.File.stderr()