After this change, any import that only takes and returns ABI-safe numbers (signed
integers less than 64 bits and unrestricted floating point numbers) will be a
direct import, and will not have a little JS shim in the middle.
We don't have a great mechanism for testing the generated bindings' contents --
as opposed to its behavior -- but I manually verified that everything here does
the Right Thing and doesn't have a JS shim:
```rust
\#[wasm_bindgen]
extern "C" {
fn trivial();
fn incoming_i32() -> i32;
fn incoming_f32() -> f32;
fn incoming_f64() -> f64;
fn outgoing_i32(x: i32);
fn outgoing_f32(y: f32);
fn outgoing_f64(z: f64);
fn many(x: i32, y: f32, z: f64) -> i32;
}
```
Furthermore, I verified that when our support for emitting native `anyref` is
enabled, then we do not have a JS shim for the following import, but if it is
disabled, then we do have a JS shim:
```rust
\#[wasm_bindgen]
extern "C" {
fn works_when_anyref_support_is_enabled(v: JsValue) -> JsValue;
}
```
Fixes#1636.
