Reland "Remove skstd::optional entirely."

This reverts commit 98e51006b9c1c8747ab99d6e2a738f64f5fc4f3b.

Flutter fix: https://github.com/flutter/engine/pull/31193

Original change's description:
> Revert "Remove skstd::optional entirely."
>
> This reverts commit 17d0fc087ced7f1deeea92de6404ce48f8433698.
>
> Reason for revert: flutter still uses the skstd class
>
> Original change's description:
> > Remove skstd::optional entirely.
> >
> > Skia now uses C++17's std::optional.
> >
> > Change-Id: I387069589baaefadd25e25bcec3f4cc6ee6fd090
> > Reviewed-on: https://skia-review.googlesource.com/c/skia/+/501477
> > Reviewed-by: Ben Wagner <bungeman@google.com>
> > Reviewed-by: Brian Osman <brianosman@google.com>
> > Commit-Queue: John Stiles <johnstiles@google.com>
> > Auto-Submit: John Stiles <johnstiles@google.com>
>
> Change-Id: I2c001588007640ac7b8c9f0760038b46c220a07e
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502702
> Auto-Submit: John Stiles <johnstiles@google.com>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>

Change-Id: Ia0cd41eda845b02f518c501a23293d344756a4eb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502703
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
55 files changed