add method to encode tuple element

[barnjamin]

I just want bytes, no need for other scratch vars or lambdas

diff from a program where i switched to this:

[ahangsu]

I am still opinionated on this one tho, after some more thoughts. Maybe I am wrong.

This encode method applies to all ComputedType, so it also applies to ReturnedType which wraps up a computation returning something on stack. What if the encode called twice for a ReturnedType instance with a computation that has side effect? That would not be ideal.

[bbroder-algo]

barnjamin can you please speak with Hang to elaborate his concerns regarding multiple instance requests of ReturnedType

[barnjamin]

@ahangsu correctly notes that if someone were to call something like encode multiple times on a ReturnedValue the computation Expr would be run again.

However, this behavior is already present in the current release. While it isn't ideal I think it shouldn't be a show stopped for this pr. Perhaps another pr that improves how a ReturnedValue caches its computation (via scratch var or otherwise?) is warranted.

[barnjamin]

I forgot about this one, assuming the conflict is resolved is it good to merge?

cc @ahangsu @bbroder-algo