wiggle: Refactor with fewer raw pointers

[alexcrichton]

This commit refactors the internals of wiggle to have fewer raw pointers and more liberally use &[UnsafeCell<_>]. The purpose of this refactoring is to more strictly thread through lifetime information throughout the crate to avoid getting it wrong. Additionally storing UnsafeCell<T> at rest pushes the unsafety of access to the leaves of modifications where Rust safety guarantees are upheld. Finally this provides what I believe is a safer internal representation of WasmtimeGuestMemory since it technically holds onto &mut [u8] un-soundly as other &mut T pointers are handed out.

Additionally generated GuestTypeTransparent impls in the wiggle macro were removed because they are not safe for shared memories as-is and otherwise aren't needed for WASI today. The trait has been updated to indicate that all bit patterns must be valid in addition to having the same representation on the host as in the guest to accomodate this.

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[abrown]

Makes sense to me. I think there are some additional changes here beyond the &[UnsafeCell<T>] one that might be worth documenting; e.g., it isn't clear to me exactly why we are removing validate here and in the future it might be good to remember the "why". Perhaps some reasoning in the squashed commit message would help?

[alexcrichton]

Ah yes indeed sorry about that. The wiggle-generated implementations of GuestTypeTransparent were all unsafe in the face of shared memories since they do raw pointer reads/writes and needed treatment somehow. One option would be to switch to atomic loads as was done for primitive integers, but I opted instead to change the definition of GuestTypeTransparent to require "all bits valid" and then I went ahead and removed the auto-generated implementations. I don't think anything needs GuestTypeTransparent, or at least not in WASI today, and it otherwise represents the ambitions-at-the-time which have since been eclipsed by the component model.