Inline context.{get,set} in components

[alexcrichton]

This commit reimplements the context.{get,set} intrinsics in the component model, introduced in the component-model-async and component-model-threading proposals. The intent of these intrinsics in WASIp3, for example, are intended to replace the globals used for the stack pointer and TLS base in previous modules, for example. The implementation of loading from a global is a single load instruction, whereas the previous implementation of context.get was a full libcall, which is significantly more expensive. The goal of this PR is to ensure that the transition to using context.get and context.set for high-performance uses retains the same performance as the WASIp2 constructs.

Specifically the storage for context.{get,set} slots have been moved into the VMStoreContext structure which has a known layout to compiled code. There still remains storage within each GuestThread because there's only one store, and the idea is that whenever threads are switched between the switch operation is slightly more expensive now where it has to update and maintain the state in the store. The rationale for this is that it'll be far more often that these values are accessed rather than threads being swapped between.

The implementation chosen in this commit is to model the context.{get,set} intrinsics as UnsafeIntrinsics. This is a bit of a shoehorn where they're not actually unsafe, but all of the plumbing and support for UnsafeIntrinsic is effectively exactly what these want. To avoid duplicating lots of infrastructure that's where these now reside.

The concurrent.rs implementation has been updated to save/restore context from the store, and this additionally updates a few other switch points to ensure that the store never switches away or to a deleted thread. This niche situation happened in a few scenarios with no impact from before, but with the switching implementation having to access threads it became load-bearing that these must be valid.

The end result is that with -Cinlining the context.{get,set} instructions are two instructions instead of a full libcall. One instruction is loading VMStoreContext, which is GVN-able and hoist-able, while the other is the actual load/store. This is the same as the performance of the stack pointer being in an imported global, for example.