[Relay][VM][Interpreter] Enable first-class constructors in VM and interpreter via eta expansion

[weberlo]

Previously, the VM and interpreter did not support constructors as first-class objects. For example, the following code snippet would not compile:

def @main[A]() -> (fn(A, List[A]) -> List[A]) {
  Cons
}

. The eta expansion pass in this PR remedies this problem by wrapping the constructor in a function, like so:

def @main[A]() -> (fn(A, List[A]) -> List[A]) {
  fn (%x: A, %xs: List[A]) -> List[A] {
    Cons(%x, %xs)  
  }
}

. There was an existing eta expansion pass, but it was not recursive and only operated on global variables. The new pass is configurable, so you can choose to expand constructors, global variables, or both.

Also included are the following minor changes:

• Fix LambdaLift pass so it closes over free type variables.
• Fix LambdaLift pass so it reuses lifted functions that are alpha equal.
• Fix pretty printing of constructors to be context-sensitive (e.g., print Cons(A, List[A]) in a type definition but only print Cons when in an expr).
• Rename Module::HasDef to Module::ContainGlobalTypeVar to match the analogous Module::ContainGlobalVar method.
• Change prelude definitions to use first-class constructors where applicable.

CC @jroesch @MarisaKirisame @icemelon9

[MarisaKirisame]

LGTM

[weberlo]

@jroesch @icemelon9 CI is green. Take a look when you get a chance.

[icemelon]

lgtm, just one minor comment

[weberlo]

It looks like the tests are timing out. I think this addition to the interpreter is the cause. Might need to roll back that change and the change to the prelude, but keep the EtaExpand functionality for the VM. If we want first-class constructors in the interpreter, we'll need to directly support it there.

[weberlo]

nvm. there seem to be CI timeouts repo-wide (https://discuss.tvm.ai/t/remove-unused-function-in-relay-module/4753)

[weberlo]

@icemelon9 CI is green again

[icemelon]

Thanks @weberlo @MarisaKirisame. This is now merged.