multiple: Make evaluation slightly less lazy and overhaul collect.

[DeedleFake]

Expressions are now evaluated when they are stored. In other words, let expressions evaluate their right-hand side when they themselves are evaluated, and function arguments are evaluated before being passed. This means that functions implemented in Go shouldn't need to manually call arg.Call(frame) on every argument before using it. A weird side-effect of this is that extra arguments passed to a function are evaluated, even if the function doesn't actually use them.

Because of this, collect no longer makes sense as a standard library function, so that function has been removed and replaced with a new syntactic structure. If (| and |) are used to delineate a compound instead of just ( and ), that compound will return its scope when evaluated instead of the last expression's result.

[DeedleFake]

Accidentally part of #166.