Add API to statically assert signature of a Func (#955)

* Add API to statically assert signature of a `Func`

This commit add a family of APIs to `Func` named `getN` where `N` is the
number of arguments. Each function will attempt to statically assert the
signature of a `Func` and, if matching, returns a corresponding closure
which can be used to invoke the underlying function.

The purpose of this commit is to add a highly optimized way to enter a
wasm module, performing type checks up front and avoiding all the costs
of boxing and unboxing arguments within a `Val`. In general this should
be much more optimized than the previous `call` API for entering a wasm
module, if the signature is statically known.

* rustfmt

* Remove stray debugging

crates/runtime/signalhandlers/Trampolines.cpp

@@ -3,35 +3,16 @@
 #include "SignalHandlers.hpp"
 
 extern "C"
-int WasmtimeCallTrampoline(
+int RegisterSetjmp(
     void **buf_storage,
-    void *vmctx,
-    void *caller_vmctx,
-    void (*trampoline)(void*, void*, void*, void*),
-    void *body,
-    void *args)
-{
+    void (*body)(void*),
+    void *payload) {
   jmp_buf buf;
   if (setjmp(buf) != 0) {
     return 0;
   }
   *buf_storage = &buf;
-  trampoline(vmctx, caller_vmctx, body, args);
-  return 1;
-}
-
-extern "C"
-int WasmtimeCall(
-    void **buf_storage,
-    void *vmctx,
-    void *caller_vmctx,
-    void (*body)(void*, void*)) {
-  jmp_buf buf;
-  if (setjmp(buf) != 0) {
-    return 0;
-  }
-  *buf_storage = &buf;
-  body(vmctx, caller_vmctx);
+  body(payload);
   return 1;
 }