Investigate a shared linker with request specific state attached to the wasmtime store

[avidal]

In bytecodealliance/wasmtime-go#260 I added an ability to attach arbitrary data to a Store. This is something that's already supported in the reference Rust implementation of wasmtime.

If (when?) this merges in, we should be able to simplify fastlike. Right now there's a lot of setup and teardown between requests so we can reuse instances out of a pool. Part of that setup and teardown is resetting state and linking instance functions into the wasm module to satisfy the ABI imports.

If we instead track per-request state on the store itself there may be opportunities to clean up fastlike. Filing this issue as a reminder to prototype it.

[avidal]

The above linked PR has been merged. Using it we can construct the store via NewStoreWithData and attach a struct for all of the various per-request state and reuse the Linker, which then allows us to drop quite a bit of the wasmctx on the Instance type and may make the entire thing more readable.

Although it will result in most of the hostcalls having to start with caller.Data().(*Session) (or whatever the struct-to-be is called) to get access to the state data which may itself look worse than what's currently in-place.