New egg: slset

Hi CHICKENeers,

The past few weeks I've been obsessing about a very niche problem,
triggered by some code in CHICKEN core that relied on SRFI-1's "lset"
procedures which treat lists as sets, specifically when those lists
contain only symbols.

The SRFI-1 lset operations are all quadratic on the input (as the docs
clearly state).  In SRFI-1 there's a throwaway remark: "One can use
algorithms based on element-marking, with linear-time results".
I figured one should be able to do this by relying on symbol plists to
hold such markings.

The result of this is the "slset" egg - treating [S]ymbol-[L]ists
as [SET]s (sorry, the name is a bit awkward).  It's available for
CHICKEN 5 *and* 6.  See the documentation at
https://wiki.call-cc.org/eggref/5/slset or
https://wiki.call-cc.org/eggref/6/slset for more info.

Cheers,
Peter