Support export ADT value in Python

[wweic]

1. Fix prelude compilation in VM. We need to optimize functions in prelude the same as entry func.
2. Fix ConstructorValueNode's tag and add optional constructor tag.

cc: @jroesch @MarisaKirisame @icemelon9 @zhiics @slyubomirsky @tqchen

[slyubomirsky]

What will be the mechanism for assigning a tag to a constructor? (Forgive me if I've missed it)

[wweic]

What will be the mechanism for assigning a tag to a constructor? (Forgive me if I've missed it)

@slyubomirsky a brute force approach would be to search through all constructors and compare tag to implement tag -> constructor mapping. Right now there is no data structure specifically for this.

I added a new function in prelude to map tag to constructor.

[zhiics]

LGTM

[icemelon]

LGTM

[jroesch]

I'm not sure if we should use a global constructor mapping upon further thought. Many implementations of tags only number the constructors for a given type. I think it might make more sense to add an optional debugging field if we want to track the constructors.

[wweic]

@jroesch agree it's better to have local tag scope for each type. I'll change it back. Do you mean add debug constructor field to ContrustorValueNode?

[wweic]

@jroesch @slyubomirsky Addressed comments. Please take another look. Thanks.

[slyubomirsky]

LGTM, thanks

[apivovarov]

@slyubomirsky Can you approve it in addition to saying LGTM?

@MarisaKirisame @icemelon9 @zhiics @slyubomirsky Can it be merged ASAP? I'm constantly getting test_sum_loop Segmentation fault errors #3352