Avoid type pirating `Base.skip`

[KristofferC]

This function seems a bit odd to define here:

https://github.com/JuliaIO/Tar.jl/blob/6bfc11475a80b752e70518047c3c3463f56bbc1d/src/Tar.jl#L12-L21

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