If pack() is called with a single value, it does not create a table; it
simply returns the value it is passed. When unpack is called with a
table argument, it interprets that table as a list of values that were
packed together into a table.
This causes a problem when the single value being packed is _itself_ a
table. pack() will not place it into another table, but unpack() sees
the table argument and tries to unpack it.
To fix this, we add a simple "tag" to packed table values so that
unpack() only attempts to unpack tables that have this tag. Other tables
are left alone. The tag is simply the length of the table.
vim.iter wraps a table or iterator function into an `Iter` object with
methods such as `filter`, `map`, and `fold` which can be chained to
produce iterator pipelines that do not create new tables at each step.
