chore(build): fix compilation warnings

[marxin]

First part of the warning clean-up, these changes make it possible to run cargo build --all without any warning.

[syrusakbary]

This should be private. Why it needs to be public?

[marxin]

The place where the type is exposed to a public API is here:

warning: type `backend::v8::vm::VMExceptionRef` is more private than the item `vm::VMExceptionRef::V8::0`
   --> lib/api/src/vm/mod.rs:22:16
    |
22  |             V8(crate::backend::v8::vm::[<VM $name>]),
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ field `vm::VMExceptionRef::V8::0` is reachable at visibility `pub`
...
445 | define_vm_like!(ExceptionRef);
    | ----------------------------- in this macro invocation
    |
note: but type `backend::v8::vm::VMExceptionRef` is only usable at visibility `pub(crate)`

[syrusakbary]

Why not in wamr?

[marxin]

Because the code down below makes use of the functions only conditionally:

wasmer/examples/memory.rs

Lines 112 to 145 in b4bff3c

	#[cfg(not(feature = "wamr"))]
	{
	// Here we are requesting two more pages for our memory.
	memory.grow(&mut store, 2)?;
	let memory_view = memory.view(&store);
	assert_eq!(memory_view.size(), Pages::from(3));
	assert_eq!(memory_view.data_size(), 65536 * 3);
	// Now that we know how to query and adjust the size of the memory,
	// let's see how wa can write to it or read from it.
	//
	// We'll only focus on how to do this using exported functions, the goal
	// is to show how to work with memory addresses. Here we'll use absolute
	// addresses to write and read a value.
	let mem_addr = 0x2220;
	let val = 0xFEFEFFE;
	set_at.call(&mut store, mem_addr, val)?;
	let result = get_at.call(&mut store, mem_addr)?;
	println!("Value at {:#x?}: {:?}", mem_addr, result);
	assert_eq!(result, val);
	// Now instead of using hard coded memory addresses, let's try to write
	// something at the end of the second memory page and read it.
	let page_size = 0x1_0000;
	let mem_addr = (page_size * 2) - mem::size_of_val(&val) as i32;
	let val = 0xFEA09;
	set_at.call(&mut store, mem_addr, val)?;
	let result = get_at.call(&mut store, mem_addr)?;
	println!("Value at {:#x?}: {:?}", mem_addr, result);
	assert_eq!(result, val);
	}