EnumeratedStream

[ІП-24 Олександр Ротань]

My proposal regarding findIndex brought up a topic that, as I see, has been
brought up here multiple times.

My idea is to extend the existing stream interface in the new
EnumeratedStream interface. Such an approach has a number of advantages.

1. Some methods will be able to explicitly specify that they require an
enumerated stream.

2. This would allow us to use an enumerated stream just as the default
value stream, if the index is redundant at a certain processing step.

3. This could help introduce a more user-friendly interface: instead of
passing pairs of index and value, they could be passed as separate
variables, which will make code more concise.

Consider following example:
List.of(1, 2, 3).stream()
                .enumerated()
                .map(idx, value -> idx % 2 == 0 ? value : -value);

looks much better than
List.of(1, 2, 3).stream()
                .enumerated()
                .map(pair -> pair.idx % 2 == 0 ? pair.value : -pair.value);

However, I see some caveats with this approach, which relate to parallel
streams:
when a stream is backed by a collection, you might expect assigned indexes
to represent order of iteration through collection, especially when talking
about sequential collections. Assigning indexes in such streams could be a
difficult task in a parallel environment. It should either assign index to
a stream elements at the moment when stream becomes parallelized, which is
also impossible if already parallelized stream is being enumerated, and
also wont work for infinite streams, or make enumerated stream at least
partially sequential by passing elements to processing in order they were
passed to stream at the first place, which is also hard or maybe even
impossible to achieve.

Also, side note: methods that already accept 2 parameters might become
kinda verbose, like reduce that will have to accept for params (idx1, val1,
idx2, val2), but i think that is not a big deal
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