first stage depends on the type passed to
`vim.iter()`:
• Lists or arrays (|lua-list|) yield only the value of each element.
  • Holes (nil values) are allowed (but discarded).
  • Use pairs() to treat array/list tables as dicts (preserve holes and
    non-contiguous integer keys): `vim.iter(pairs(…))`.
  • Use |Iter:enumerate()| to also pass the index to the next stage.
    • Or initialize with ipairs(): `vim.iter(ipairs(…))`.
• Non-list tables (|lua-dict|) yield both the key and value of each element.
• Function |iterator|s yield all values returned by the underlying function.
• Tables with a |__call()| metamethod are treated as function iterators.

The iterator pipeline terminates when the underlying |iterable| is exhausted
(for function iterators this means it returned nil).

Note: `vim.iter()` scans table input to decide if it is a list or a dict; to
avoid this cost you can wrap the table with an iterator e.g.
`vim.iter(ipairs({…}))`, but that precludes the use of |list-iterator|
operations such as |Iter:rev()|).

Examples: >lua
    local it = vim.iter({ 1, 2, 3, 4, 5 })
    it:map(function(v)
      return v * 3
    end)
    it:rev()
    it:skip(2)
    it:totable()
    -- { 9, 6, 3 }

    -- ipairs() is a function iterator which returns both the index (i) and the value (v)
    vim.iter(ipairs({ 1, 2, 3, 4, 5 })):map(function(i, v)
      if i > 2 then return v end
    end):totable()
    -- { 3, 4, 5 }

    local it = vim.iter(vim.gsplit('1,2,3,4,5', ','))
    it:map(function(s) return tonumber(s) end)
    for i, d in it:enumerate() do
      print(string.format("Column %d is %d", i, d))
    end
    -- Column 1 is 1
    -- Column 2 is 2
    -- Column 3 is 3
    -- Column 4 is 4
    -- Column 5 is 5

    vim.iter({ a = 1, b = 2, c = 3, z = 26 }):any(function(k, v)
      return k == 'z'
    end)
    -- true

    local rb = vim.ringbuf(3)
    rb:push("a")
    rb:push("b")
    vim.iter(rb):totable()
    -- { "a", "b" }
<


Iter:all({pred})                                                  *Iter:all()*
    Returns true if all items in the iterator match the given predicate.

    Parameters: ~
      • {pred}  (`fun(...):boolean`) Predicate function. Takes all values
                returned from the previous stage in the pipeline as arguments
                and returns true if the predicate matches.

Iter:any({pred})                                                  *Iter:any()*
    Returns true if any of the items in the iterator match the given
    predicate.

    Parameters: ~
      • {pred}  (`fun(...):boolean`) Predicate function. Takes all values
                returned from the previous stage in the pipeline as arguments
                and returns true if the predicate matches.

Iter:each({f})                                                   *Iter:each()*
    Calls a function once for each item in the pipeline, draining the
    iterator.

    For functions with side effects. To modify the values in the iterator, use
    |Iter:map()|.

    Parameters: ~
      • {f}  (`fun(...)`) Function to execute for each item in the pipeline.
             Takes all of the values returned by the previous stage in the
             pipeline as arguments.

Iter:enumerate()                                            *Iter:enumerate()*
    Yields the item index (count) and value for each item of an iterator
    pipeline.

    For list tables, this is more efficient: >lua
        vim.iter(ipairs(t))
<

    instead of: >lua
        vim.iter(t):enumerate()
<

    Example: >lua

        local it = vim.iter(vim.gsplit('abc', '')):enumerate()
        it:next()
        -- 1	'a'
        it:next()
        -- 2	'b'
        it:next()
        -- 3	'c'
<

    Return: ~
        (`Iter`)

Iter:filter({f})                                               *Iter:filter()*
    Filters an iterator pipeline.

    Example: >lua
        local bufs = vim.iter(vim.api.nvim_list_bufs()):filter(vim.api.nvim_buf_is_loaded)
<

    Parameters: ~
      • {f}  (`fun(...):boolean`)