lambda list parsing w/ allow-specializers uses constantp inappropriately
`(parse-ordinary-lambda-list '((foo t)) :allow-specializers t)` works no problem. But `(parse-ordinary-lambda-list '((symbol-name t)) :allow-specializers t)` pops up an error that `(symbol-name t)` is not a valid variable name, at least on SBCL. Why? Because `parse-ordinary-lambda-list` checks not `(constantp 'symbol-name)`, like it's probably supposed to, but instead checks `(constantp '(symbol-name t))`, which is true on SBCL. There are two obvious solutions. First, the function could be changed to check `constantp` of just the variable name, when a specializer is provided. Secondly, and what I personally would prefer, the `constantp` check could be cut out entirely. It's more of a semantic check than syntactic, and users may want a lambda list parsed regardless and to handle constancy themselves, especially if they're doing something a bit exotic like compiling in alternate environments. But either fix would work for me.
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