[parser,ast] Allow function captures as block parameters for instantiation

[faultyserver]

Instantiation should support all of the syntax features of a normal call. This includes using a function capture as the block parameter when it is the last argument of the instantiation.

While working on improving the Spec library, I ran into a semantic error when trying to pass through a block parameter to an instantiation. The capture was treated as a regular argument, meaning the instantiation did not match any initializers, which were expecting block arguments.

An example of this usage:

deftype Thing
  def initialize(&block)
    # Storing a block for later use...already supported
    @block = &block
  end

  defstatic create(&block)
    # Passthrough with a capture as a block...would cause "No matching clause"
    %Thing{&block}
  end
end

Thing.create{ IO.puts(:hi) }