Handle pointer arguments in lifting

[chc4]

Related to #2, but on the lifting side: if we have a closure like so:

struct MyStruct {
    a: usize,
    b: usize,
    c: usize
}

jit.speedup(|env: MyStruct| { env.a += 1; env.a })

it's implemented as something like fn(clos: c_void, env: &MyStruct) { env->a++; env->a }: that is, the structure is behind a pointer (we optimize away all loads from clos, which is the closed environment).

We want to tell the lifter that the argument is a JitValue::Ref(JitValue::Struct(vec![JitValue::Value, JitValue::Value, JitValue::Value]), mem::size_of<MyStruct>()) - that is, that it's a pointer to a struct made up of some symbolic value members. This would allow us to do partial application and optimization with the struct's members instead of having to emit Cranelift load/stores for everything.

We might want to annotate the struct MyStruct with a #[derive(JitInput)] or something? Dunno.