inspired by Chez Scheme's "Efficient Gensym Hack" (and mostly to get
familiar with the SBCL build process), I hacked up gensym to delay
consing up of a symbol-name until it is actually requested.
Except for micro-benchmarks the savings are modest, unsurprisingly.
In part, this is also due to Spec requirements wrt. *gensym-counter*.
The main beneficiary seems to be sb-ext:*evaluator-mode* :interpret,
but even there I couldn't make a compelling case (at least not with
cl-bench) of improvement vs. the added complexity.
Thus, I decided to post the patch as-is, and not really for inclusion
into SBCL. Maybe somebody else wants to run with it.