Extend @typeParam to express lower bound on type parameter

[jclark]

Our existing @typeParam annotation isn't quite powerful enough to capture the type of stream:toArray. It might seem like you could write:

function toArray(stream<Type,ErrorType?> stm) returns Type[]|ErrorType;

But this isn't quite right, because stream<T,error> is a subtype of stream<T,error?>, so this would allow you to pass an unbounded stream. It would also not be right to say:

function toArray(stream<Type,()> stm) returns Type[];

because that wouldn't allow a bounded stream that might return an error.

When the lang library does:

@typeParam
type XType X;
public function foo(XType t) returns XType = external;

X is the upper bound on the type parameter XType i.e. the type parameter can only be instantiated with a type T that is <= XType (i.e. with a subtype of XType). But for toArray what we need is a lower bound on the type, where the lower bound in this case is nil i.e. that it can only be instantiated with a type T such that () <= T. This is a lower bound that is in addition to the upper bound of error?.

So we will need to devise a way to represent that with an annotation in lang lib.

Originally posted by @jclark in #453 (comment)

[jclark]

We have this problem with stream:reduce already.