syntax-rules special literals

[alexshinn]

... and with the result of #6 also _ have special meaning
in syntax-rules patterns, so they are not treated as pattern
variables by default.

However their behavior when used in the literals list of
syntax-rules is ambiguous, and simply breaks in most implementations.

Rather than breaking, it makes sense to go ahead and treat
them as normal literals, overriding their special meanings.

In particular, there are many existing R5RS macros which
make use of _ in the literals and are thus broken outright
by #6. Allowing them as literals fixes these macros.

[cowan]

WG1 chose to make them literals.