roll: pigweed pw_allocator: Add conversions for managed pointers

This CL adds additional ways to convert managed pointers of one type to
another, in order to make them more flexible and usable by downstream
projects.

* Explicit conversion operators are added to allow downcasting.
* A SharedPtr constructor that takes a UniquePtr is added.
* Some missing comparison operators were added to aid testing.

Original-Bug: 432456663
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/317224
Original-Revision: 9404c71b7d35d364b35ac616eea6c8766ec40358

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: b22a59a7ce927c..9404c71b7d35d3
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8703502838281037521
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia89344f62a81ef94b2c6b89c03bd4cc92ef2fc0b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/showcase/sense/+/323012
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
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