lazily evaluate sort of KItem

@yilongli please review

I'm not going to run the complete suite of performance info again, but here is the before/after for collatz n=100

Before:
[210220, 9.495 s]
7.772 s(mc=1.266 s, eval=4.421 s[3.261 s, 1.053 s], rew=813.1 ms) + 724.3 ms
564.6 ms
17.66 ms

After:
[210220, 7.209 s]
5.862 s(mc=947.7 ms, eval=3.379 s[2.352 s, 940.2 ms], rew=744.7 ms) + 536.9 ms
459.9 ms
19.94 ms

Note that this compares favorably as well to the statistics from before Andrei's refactoring of lookups: 
[210220, 9.284 s]
7.193 s(mc=1.262 s, eval=3.663 s[356.0 ms, 1.519 s], rew=1.294 s) + 975.0 ms
617.1 ms
19.14 ms

The numbers aren't directly comparable because we're timing different regions, but note that while it is still a little more expensive doing lookups as function evaluations, the overall time has improved and is now only marginally slower than before after taking into account the difference in requires time.