opt: reduce allocations in OrderingSet.LongestCommonPrefix

[mgartner]

Previously, the *OrderingChoice returned by LongestCommonPrefix was
allocated on the heap (except in the case where the ordering set
contained a ordering choice that implies the given ordering). Now,
LongestCommonPrefix returns a non-pointer type and avoids these
allocations.

Release justification: This is a minor change that improves performance.

Release note: None

[cockroach-teamcity]

This change is 

[mgartner]

This provides a fairly significant improvement to the slow queries benchmarks:

name                         old time/op    new time/op    delta
SlowQueries/slow-query-1-24    35.4ms ± 1%    35.3ms ± 0%     ~     (p=0.310 n=5+5)
SlowQueries/slow-query-2-24     549ms ± 0%     495ms ± 1%   -9.78%  (p=0.008 n=5+5)

name                         old alloc/op   new alloc/op   delta
SlowQueries/slow-query-1-24    12.3MB ± 0%    12.3MB ± 0%   -0.29%  (p=0.008 n=5+5)
SlowQueries/slow-query-2-24     181MB ± 0%     155MB ± 0%  -14.04%  (p=0.008 n=5+5)

name                         old allocs/op  new allocs/op  delta
SlowQueries/slow-query-1-24      131k ± 0%      130k ± 0%   -0.57%  (p=0.008 n=5+5)
SlowQueries/slow-query-2-24     1.65M ± 0%     1.24M ± 0%  -24.66%  (p=0.008 n=5+5)

[rytaft]

Wow nice improvement!

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball and @michae2)

[mgartner]

TFTR!

bors r+

[craig]

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[craig]

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[craig]

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