Ticket 238: Reserve #! for read directives

From Denis Washington:

Reading chapter 2 of the third draft, I was thinking: now that we have #!fold-case and #!no-fold-case and other directives might follow in WG2, wouldn't it be appropriate for section 2.3 (Other notations) to define #! as generally introducing a "read directive"? That would encourage implementations to use the same syntax for their own directives, which helps portability (an implementation could just ignore unknown directives which might just be used by another for optimization purposes). Just an idea.