Add non-slurping candidates for various list subs.

Prevent an extra layer of wrapping, and way more likely to inline. For
map on a 5-element list in a loop, around a 5% saving.
Optimize coercion to Order enum.

In the long run, it'd be good to emit better code for coercion to an
enum type. For now, this hot-paths it in cmp and <=>, which are used
in a whole range of operations. Knocked 15% off a sort benchmark.
Micro-opt on sort sub.