Define names and lengths for iterator functions

Define names and lengths for iterator functions

Iterator function generators had names, but the functions they
generated did not. Now they do, following the general conventions of
existing functions like `Array::[Symbol.iterator]` (not that these
_particularly_ make sense). Their lengths were already 1 and they were
already not constructable, but this is now tested.

Also, minor changes to documentation, ⁊·c.