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>
54 files changed