feat: per call context (#711)

Add `plugin.call_with_host_context` and `current_plugin.host_context`
methods, enabling per-call context to be looped from the guest invocation
to any host functions it calls. In an HTTP server environment, this enables
re-using a plugin across multiple requests while switching out backing
connections and user information in host functions. (Imagine a host
function, `update_user` -- previously the plugin would have to have been
aware of the user to pass to the host function. Now that information is
ambient.)

This is a backwards-compatible change and requires no changes to
existing plugins.

Implement by adding a global, mutable externref to the extism kernel.
Since most programming languages, including Rust, don't let you define
these natively, we accomplish this by using `wasm-merge` to combine the
kernel Wasm with Wasm generated by a WAT file containing only the
global.

(This pattern might be useful for other Wasm constructs we can't use
directly from Rust, like `v128` in argument parameters.)

Wasmtime requires extern refs to be `Any + Send + Sync + 'static`; we
additionally add `Clone`. I haven't tried this with an `Arc` directly,
but it should work at least for container structs that hold `Arc`'s
themselves.

kernel/build.sh

@@ -17,6 +17,9 @@ done
 
 cargo build --package extism-runtime-kernel --bin extism-runtime --release --target wasm32-unknown-unknown $CARGO_FLAGS
 cp target/wasm32-unknown-unknown/release/extism-runtime.wasm .
-wasm-strip extism-runtime.wasm
-mv extism-runtime.wasm ../runtime/src/extism-runtime.wasm
 
+wasm-tools parse extism-context.wat -o extism-context.wasm
+wasm-merge --enable-reference-types ./extism-runtime.wasm runtime extism-context.wasm context -o ../runtime/src/extism-runtime.wasm
+rm extism-context.wasm
+rm extism-runtime.wasm
+wasm-strip ../runtime/src/extism-runtime.wasm

kernel/extism-context.wat

@@ -0,0 +1,3 @@
+(module
+  (global (export "extism_context") (mut externref) (ref.null extern))
+)