refactor: refactor Struct and StructValue

[ZENOTME]

as #135 (comment) says, we should keep type info in struct to be Arc so that these info can be shared if we have many values to reduce memory cost.

This PR:

1. refactor Struct: Hide this field and expose interface to access it/
2. refactor StructValue: hold the type info using Arc<Struct>
3. Make the Struct in Any to be Arc so that we can share the type info when create struct value.

enum Any {
    Struct(Arc<Struct>)
}

[ZENOTME]

There are two todo check here:

1. for check field type, Maybe we need to add interface for AnyValue to return the according type like:

impl AnyValue {
    fn return_type(&self) -> Any 
}

2. Not sure whether we need it🤔

[liurenjie1024]

I think we can postpone the check later.

[liurenjie1024]

Thanks, LGTM