Ticket 447: #!fold-case and #!no-fold-case have no final delimiter

The #!fold-case and #!no-fold-case directives are read as comments, which means that they are treated as whitespace (section 2.2). Unlike the other kinds of comments, their final delimiter is implicit. This means that (1#!no-fold-cases) reads as (1 s). This seems unfortunate.

Current proposals:

1. Add the formal syntax <lexical-directive> --> #! <identifier> and then make the interpretation of <identifier> implementation-dependent, except for the standard cases #!fold-case and #!no-fold-case. (Per Bothner, Richard Kelsey)
2. Followed by whitespace (John Cowan)