feat: add support for WASIp1 programs (Flatware)

[Akshay-Srivatsan]

This is a bit of an omnibus PR since I kept running into random things I meant to clean up anyway, but roughly this PR:

1. Documents Arca, Fix, and Flatware's intended designs.
2. Locks the Handle code to a specific bit representation rather than auto-deriving it via BitPack. The representation is almost what we designed for the old Fix, but it's extended for the three-way canonical/machine/local distinction and for the Eq bit. The Handles are now uniformly represented as either [u8; 32] (kernel) or u8x32 (userspace); we no longer unpack them into a Rust enum.
3. Implements Flatware, roughly based on Haibib's and Saibi's "Arcware" design from the summer. I made a few tweaks to make some things simpler (mostly merging the filesystem and descriptor tables since WASIp1 doesn't really distinguish them anyway).
4. Adds C/C++/Rust "SDKs" for writing Fix functions. The C SDK is basically just the raw Fix Wasm API, but the C++ and Rust SDKs expose a more-or-less "normal" Fix ontology. Programs interact with Handles to Objects, Ref, Thunks, and Encodes; these operations are lazy and build up an expression AST, then call out to a C interpreter when they actually need data to give to the client program.
5. Implements the missing eq bit based on rules which I think are sound. We should probably prove this... I couldn't find a concrete equality definition in the coupon proof that matched what makes sense to me operationally.
6. I think I probably added more functionality to the Fix Shell in the process of doing the rest of this?

Still TBD:

1. Continuation support in Fix.
2. Connecting the coupon collector to the actual Fix API and scheduler.
3. In-process local handles backed by virtual addresses.
4. Any sort of on-disk persistence of Fix values.

With my apologies to anyone who tries to review this 14000 line diff... this PR adds almost a third to the codebase. Although at least it's only like 7000 lines of actual code? And the final total is now about the same as the original Fix codebase?

[Akshay-Srivatsan]

For what it's worth each commit on this branch is meant to be an independent change, GitHub just won't let me make it a PR stack unless I create a new branch for each one.