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()
and rename std.io.BufferedWriter.writableSlice to writableSliceGreedy
and make writableSlice and writableArray advance the buffer end position
introduce std.io.BufferedWriter.writeSplatLimit but it's unimplemented
remove std.fs.Dir.readFileAllocOptions, replace with more flexible API
readFileIntoArrayList
remove std.fs.File.readToEndAllocOptions, replace with more flexible API
readIntoArrayList
update std.fs.File to new reader/writer API
add helper functions to std.io.Reader.Limit
replace std.io.Writer.FileLen with std.io.Reader.Limit
make offset a type rather than u64 so that it can distinguish between
streaming read and positional read
avoid an unnecessary allocation in std.zig.readSourceFileToEndAlloc when
there is a UTF-16 little endian BOM.
Simplifies code in docs creation where we used `std.tar.output.Header`.
Writer uses that Header internally and provides higher level interface.
Updates checksum on write, handles long file names, allows setting mtime and file permission mode. Provides handy interface for passing `Dir.WalkerEntry`.
Now that `-femit-docs` includes all modules, including the builtin
module, in the generated source tarball, it makes sense to apply the
same logic to the std-docs server. std-docs constructs its own tarball,
so a different approach is needed to achieve the same end result.
