* move std.io.File to std.os.File
* add `zig fmt` to self hosted compiler
* introduce std.io.BufferedAtomicFile API
* introduce std.os.AtomicFile API
* add `std.os.default_file_mode`
* change FileMode on posix from being a usize to a u32
* add std.os.File.mode to return mode of an open file
* std.os.copyFile copies the mode from the source file instead of
using the default file mode for the dest file
* move `std.os.line_sep` to `std.cstr.line_sep`
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