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/api/src/lib.rs

@@ -24,7 +24,7 @@ mod values;
 pub use crate::callable::Callable;
 pub use crate::externals::*;
 pub use crate::frame_info::FrameInfo;
-pub use crate::func::{Func, WasmArg, WasmRet};
+pub use crate::func::{Func, WasmRet, WasmTy};
 pub use crate::instance::Instance;
 pub use crate::module::Module;
 pub use crate::r#ref::{AnyRef, HostInfo, HostRef};