Support tuple assign

[jonchoi]

Can't return (num, num, bytes) without bytes being limited, but can't return bytes < 1024 as part of a tuple.

This is related to the discussion on structs / tuples (#602)

[jonchoi]

Talked with @DavidKnott, this might actually be that Viper doesn't support:

x, y = self.returns_tuple()

What's the suggested pattern for doing something like this?

[DavidKnott]

@jonchoi I think it makes sense to consider adding support for:

x: type
y: type
x, y = self.returns_tuple()

Down the road

[jonchoi]

#545
#596

[jacqueswww]

@jonchoi at the point in time, it was decided to only support returning tuple (to stay compatible with solidity contracts that do the same). +1 for adding support for tuple assignment (I really like it from python hehe).

[jacqueswww]

Assignment to tuple is now supported 😄