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)

test/functional/lua/vim_spec.lua

@@ -3374,13 +3374,18 @@ describe('lua stdlib', function()
     end)
 
     it('handles map-like tables', function()
-      local t = { a = 1, b = 2, c = 3 }
-      local it = vim.iter(t):map(function(k, v)
+      local it = vim.iter({ a = 1, b = 2, c = 3 }):map(function(k, v)
         if v % 2 ~= 0 then
           return k:upper(), v * 2
         end
       end)
-      eq({ A = 2, C = 6 }, it:totable())
+
+      local t = it:fold({}, function(t, k, v)
+        t[k] = v
+        return t
+      end)
+      eq({ A = 2, C = 6 }, t)
+    end)
 
     it('handles table values mid-pipeline', function()
       local map = {