An early exit from a loop over an iterator-typed Map or Set method parks
the iterator with a second yield of the current entry. That yield read the
entry again by its index, which aborted with a map iteration error when
the loop had deleted the entry. Each step now reads the entry once into a
local that both yields answer.
`entries()` takes the typed walk only for a slot whose yield type is the
key and value pair record. Other yield types, such as
`IterableIterator<any>` or `<unknown>`, keep the checked adapter instead of
failing to compile.
Chris Tate
c296362834