Avoid type pirating `Base.skip`

[KristofferC]

This function seems a bit odd to define here:

Tar.jl/src/Tar.jl

Lines 12 to 21 in 6bfc114

	function Base.skip(io::Union{Base.Process, Base.ProcessChain}, n::Integer)
	n < 0 && throw(ArgumentError("cannot skip backwards when reading from a process"))
	isempty(skip_buffer) && resize!(skip_buffer, DEFAULT_BUFFER_SIZE)
	while n > 0
	n -= readbytes!(io, skip_buffer, min(n, length(skip_buffer)))
	end
	# TODO: our skip data should throw if there's not enough data
	return io
	end
	const skip_buffer = UInt8[]

It also has some issues, like referencing a global skip_buffer (so the function is not thread-safe).