Build `Observation` in Embedded mode

[rauhul]

Let's get the observation library working in embedded mode if possible!

Open questions:

• KeyPath usage
• Thread Local Storage

[kubamracek]

Just some observations (heh):

• @Observation macro expands to calls to ObservationRegistrar's .access, .willSet, .didSet, .withMutation, which all currently need an input KeyPath argument. KeyPath-as-a-runtime-value is currently not available in Embedded Swift, only compile-time foldable KeyPaths are currently allowed.
• There don't seem to be any use of any / existentials in the macro expansions, or the Observation runtime, so we're good there.
• The Observation runtime doesn't use reflection.
• The Observation runtime currently doesn't build as an Embedded Swift module. It's a mixed Swift and C++ module. We are only able currently to build C++ parts into separate libraries and that means an extra step for users (to link in the library). Maybe we could find a way to make a pure Swift library?
• The Observation runtime uses KeyPaths (see above) but also the type-erased AnyKeyPath.
• The Observation runtime uses some threading facilities namely locks and thread-local storage. Those will introduce platform dependencies in non-single-threaded build (based on SWIFT_THREADING_PACKAGE).

[sliemeobn]

a few thoughts for your consideration since I just made a (hopefully temporary) embedded-compatible port of Observation (source code here). very untested, but seems to work ok.

then main issue for me was the AnyKeyPath, so I used the macro to generate hashable, typeless PropertyIDs instead. I was poking around to see if I could get any type-erased, hashable thing out of the KeyPaths (like a memory offset or something), but I did not find anything useful.

I also have not yet tackled thread-locals and locking, but I figured I'd leave these puzzles for you guys. (I was hoping Mutex would become supported eventually somehow)