chore: use Arc instead of lifetime parameter

[mohanson]

This PR modified the definition of AsmMachine:

// Before
pub struct AsmMachine<'a> {
    pub machine: DefaultMachine<'a, Box<AsmCoreMachine>>,
    pub aot_code: Option<&'a AotCode>,
}

// After
pub struct AsmMachine {
    pub machine: DefaultMachine<Box<AsmCoreMachine>>,
    pub aot_code: Option<Arc<AotCode>>,
}

This allows us to completely remove any lifetime parameter in ckb-vm.

[xxuejie]

I think it's better to use Arc here. It's not that ckb-vm will be using threads, it's more that if you think about the usage patterns, it's likely the same binary being AOTed could be shared amongst multiple ckb-vm instances, all using the same AotCode instance to speed up execution. In this case, AotCode might be shared among multiple threads. Using Rc here severely limits the ability to share compiled immutable code.

[mohanson]

Right.

[mohanson]

CC @xxuejie