RFE: Refactor java.util.Optional and add NonNull checks

[Oleksii Kucheruk]

Hi there.
I have found that `java.util.Optional` is written procedural style and has
`ifnonnull`  checks in each method. I propose to refactor `Optional` in
accordance to OOP-style. This will eliminates all unnecessary
`if`-statements, removes duplications and reduces bytecode size more then
twice.

I have two solutions:
1. Completely dynamic that avoids single static `EMPTY` instance and
unchecked casting of each `Optional.empty()`
2. Preserving original single static `EMPTY` per VM.

Also there are couple methods that throws NPE due to calling methods on
null-objects and requires to add `Objects.requireNonNull(...)`.

OptionalInt, OptionalDouble, OptionalLong could be refactored same way even
with remove of additional boolean variable `isPresent`.

Since I'm new here any guidance will be helpful.
Thank you in advance.
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