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.
