Retain unsafe in imports

[daxpedda]

I noticed that unsafe wasn't transferred when I specified it in imports, meaning the generated functions didn't have an unsafe specifier on them.

#[wasm_bindgen]
extern "C" {
	type Foo;

	#[wasm_bindgen(method, js_name = foo)]
	unsafe fn foo(&Test);
}

let foo: Foo = ...;
// Can be called without an `unsafe` block.
foo.foo();

This was particularly helpful when I was importing functions that were messing with this instances memory, which is actually unsafe.