Reactor support. (#1565)

* Reactor support.

This implements the new WASI ABI described here:

https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md

It adds APIs to `Instance` and `Linker` with support for running
WASI programs, and also simplifies the process of instantiating
WASI API modules.

This currently only includes Rust API support.

* Add comments and fix a typo in a comment.

* Fix a rustdoc warning.

* Tidy an unneeded `mut`.

* Factor out instance initialization with `NewInstance`.

This also separates instantiation from initialization in a manner
similar to https://github.com/bytecodealliance/lucet/pull/506.

* Update fuzzing oracles for the API changes.

* Remove `wasi_linker` and clarify that Commands/Reactors aren't connected to WASI.

* Move Command/Reactor semantics into the Linker.

* C API support.

* Fix fuzzer build.

* Update usage syntax from "::" to "=".

* Remove `NewInstance` and `start()`.

* Elaborate on Commands and Reactors and add a spec link.

* Add more comments.

* Fix wat syntax.

* Fix wat.

* Use the `Debug` formatter to format an anyhow::Error.

* Fix wat.

crates/runtime/src/instance.rs

@@ -1330,8 +1330,4 @@ pub enum InstantiationError {
     /// A trap ocurred during instantiation, after linking.
     #[error("Trap occurred during instantiation")]
     Trap(Trap),
-
-    /// A trap occurred while running the wasm start function.
-    #[error("Trap occurred while invoking start function")]
-    StartTrap(Trap),
 }