Remove another thread local in instance.rs (#862)

* Remove another thread local in `instance.rs`

This commit removes another usage of `thread_local!` in the continued
effort to centralize all thread-local state per-call (or basically state
needed for traps) in one location. This removal is targeted at the
support for custom signal handlers on instances, removing the previous
stack of instances with instead a linked list of instances.

The `with_signals_on` method is no longer necessary (since it was always
called anyway) and is inferred from the first `vmctx` argument of the
entrypoints into wasm. These functions establish a linked list of
instances on the stack, if needed, to handle signals when they happen.

This involved some refactoring where some C++ glue was moved into Rust,
so now Rust handles a bit more of the signal handling logic.

* Update some inline docs about `HandleTrap`

crates/jit/src/action.rs

@@ -190,14 +190,12 @@ pub fn invoke(
     // Call the trampoline. Pass a null `caller_vmctx` argument as `invoke` is
     // all about calling from the outside world rather than from an instance.
     if let Err(trap) = unsafe {
-        instance.with_signals_on(|| {
-            wasmtime_call_trampoline(
-                callee_vmctx,
-                ptr::null_mut(),
-                exec_code_buf,
-                values_vec.as_mut_ptr() as *mut u8,
-            )
-        })
+        wasmtime_call_trampoline(
+            callee_vmctx,
+            ptr::null_mut(),
+            exec_code_buf,
+            values_vec.as_mut_ptr() as *mut u8,
+        )
     } {
         return Ok(ActionOutcome::Trapped(trap));
     }