fix(iter): remove special case totable for map-like tables

This was originally meant as a convenience but prevents possible
functionality. For example:

  -- Get the keys of the table with even values
  local t = { a = 1, b = 2, c = 3, d = 4 }
  vim.iter(t):map(function(k, v)
    if v % 2 == 0 then return k end
  end):totable()

The example above would not work, because the map() function returns
only a single value, and cannot be converted back into a table (there
are many such examples like this).

Instead, to convert an iterator into a map-like table, users can use
fold():

  vim.iter(t):fold({}, function(t, k, v)
    t[k] = v
    return t
  end)

runtime/lua/vim/iter.lua

@@ -18,14 +18,6 @@ ListIter.__call = function(self)
   return self:next()
 end
 
---- Special case implementations for iterators on non-list tables.
----@class TableIter : Iter
-local TableIter = {}
-TableIter.__index = setmetatable(TableIter, Iter)
-TableIter.__call = function(self)
-  return self:next()
-end
-
 ---@private
 local function unpack(t)
   if type(t) == 'table' and t.__n ~= nil then
@@ -185,10 +177,10 @@ end
 
 --- Collect the iterator into a table.
 ---
---- The resulting table depends on the initial source in the iterator pipeline. List-like tables
---- and function iterators will be collected into a list-like table. If multiple values are returned
---- from the final stage in the iterator pipeline, each value will be included in a table. If a
---- map-like table was used as the initial source, then a map-like table is returned.
+--- The resulting table depends on the initial source in the iterator pipeline.
+--- List-like tables and function iterators will be collected into a list-like
+--- table. If multiple values are returned from the final stage in the iterator
+--- pipeline, each value will be included in a table.
 ---
 --- Examples:
 --- <pre>lua
@@ -199,9 +191,13 @@ end
 --- -- { { 1, 2 }, { 2, 4 }, { 3, 6 } }
 ---
 --- vim.iter({ a = 1, b = 2, c = 3 }):filter(function(k, v) return v % 2 ~= 0 end):totable()
---- -- { a = 1, c = 3 }
+--- -- { { 'a', 1 }, { 'c', 3 } }
 --- </pre>
 ---
+--- The generated table is a list-like table with consecutive, numeric indices.
+--- To create a map-like table with arbitrary keys, use |Iter:fold()|.
+---
+---
 ---@return table
 function Iter.totable(self)
   local t = {}
@@ -239,17 +235,21 @@ function ListIter.totable(self)
   return Iter.totable(self)
 end
 
----@private
-function TableIter.totable(self)
-  local t = {}
-  for k, v in self do
-    t[k] = v
-  end
-  return t
-end
-
 --- Fold an iterator or table into a single value.
 ---
+--- Examples:
+--- <pre>
+--- -- Create a new table with only even values
+--- local t = { a = 1, b = 2, c = 3, d = 4 }
+--- local it = vim.iter(t)
+--- it:filter(function(k, v) return v % 2 == 0 end)
+--- it:fold({}, function(t, k, v)
+---   t[k] = v
+---   return t
+--- end)
+--- -- { b = 2, d = 4 }
+--- </pre>
+---
 ---@generic A
 ---
 ---@param init A Initial value of the accumulator.
@@ -783,7 +783,7 @@ function Iter.new(src, ...)
       count = count + 1
       local v = src[count]
       if v == nil then
-        return TableIter.new(src)
+        return Iter.new(pairs(src))
       end
       t[count] = v
     end
@@ -827,25 +827,4 @@ function ListIter.new(t)
   return it
 end
 
---- Create a new TableIter
----
----@param t table Table to iterate over. For list-like tables, use ListIter.new instead.
----@return Iter
----@private
-function TableIter.new(t)
-  local it = {}
-
-  local index = nil
-  function it.next()
-    local k, v = next(t, index)
-    if k ~= nil then
-      index = k
-      return k, v
-    end
-  end
-
-  setmetatable(it, TableIter)
-  return it
-end
-
 return Iter