Store WasmFuncType in FuncType (#2365)

This commit updates `wasmtime::FuncType` to exactly store an internal
`WasmFuncType` from the cranelift crates. This allows us to remove a
translation layer when we are given a `FuncType` and want to get an
internal cranelift type out as a result.

The other major change from this commit was changing the constructor and
accessors of `FuncType` to be iterator-based instead of exposing
implementation details.

examples/multi.rs

@@ -22,8 +22,8 @@ fn main() -> Result<()> {
     // Create external print functions.
     println!("Creating callback...");
     let callback_type = FuncType::new(
-        Box::new([ValType::I32, ValType::I64]),
-        Box::new([ValType::I64, ValType::I32]),
+        [ValType::I32, ValType::I64].iter().cloned(),
+        [ValType::I64, ValType::I32].iter().cloned(),
     );
     let callback_func = Func::new(&store, callback_type, |_, args, results| {
         println!("Calling back...");

examples/threads.rs

@@ -8,18 +8,14 @@ use wasmtime::*;
 const N_THREADS: i32 = 10;
 const N_REPS: i32 = 3;
 
-fn print_message(_: Caller<'_>, args: &[Val], _: &mut [Val]) -> Result<(), Trap> {
-    println!("> Thread {} is running", args[0].unwrap_i32());
-    Ok(())
-}
-
 fn run(engine: &Engine, module: Module, id: i32) -> Result<()> {
     let store = Store::new(&engine);
 
     // Create external print functions.
     println!("Creating callback...");
-    let callback_type = FuncType::new(Box::new([ValType::I32]), Box::new([]));
-    let callback_func = Func::new(&store, callback_type, print_message);
+    let callback_func = Func::wrap(&store, |arg: i32| {
+        println!("> Thread {} is running", arg);
+    });
 
     let id_type = GlobalType::new(ValType::I32, Mutability::Const);
     let id_global = Global::new(&store, id_type, Val::I32(id))?;