Change iteration protocol
This changes the iteration protocol from `start`/`next`/`done` to `iterate`.
The new lowering of a for loop is as follows:
```
for x in itr
...
end
```
becomes
```
next = iterate(itr)
while next !== nothing
x, state = next::Tuple{Any, Any}
...
next = iterate(itr, state)
end
```
The semantics are as apparent from the above lowering. `iterate` returns
either `nothing` or a tuple of value and state. The state is passed
to any subsequent operation. The first iteration is indicated, by not passing
the second, state argument to the `iterate` method.
Adaptors in both directions are provided to keep the legacy iteration
protocol working for now. However, performance of the legacy iteration
protocol will be severely pessimized.
As an optional add-on for mutable iterators, a new `isdone` function is
provided. This function is intended as an O(1) approximate query for
iterator completion, where such a calculation is possible without mutation
and/or is significantly faster than attempting to obtain the element itself.
The function makes use of 3-value logic. `missing` is always an acceptable
answer, in which case the caller should go ahead and attempt the iteration
to obtain a definite result. If the result is not `missing`, it must be
exact (i.e. if true, the next call to iterate must return `nothing`, if
false it must not return nothing).
