avoid boxing

[xuwei-k]

http://stackoverflow.com/a/16112715
scala/scala@26c87af

[codecov-io]

Codecov Report

Merging #646 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
- Coverage      50%   49.97%   -0.03%     
==========================================
  Files         173      173              
  Lines       11167    11167              
  Branches     1548     1566      +18     
==========================================
- Hits         5584     5581       -3     
- Misses       5583     5586       +3

Impacted Files	Coverage Δ
...cala/spire/math/extras/interval/IntervalTrie.scala	0% <0%> (ø)	⬆️
...d/src/main/scala_2.10/spire/util/OptVersions.scala	0% <0%> (-66.67%)	⬇️
...e/shared/src/main/scala/spire/math/Algebraic.scala	67.35% <0%> (-1.65%)	⬇️
...ore/shared/src/main/scala/spire/math/Natural.scala	58.46% <0%> (-0.93%)	⬇️
extras/src/main/scala/spire/math/FixedPoint.scala	36.06% <0%> (-0.55%)	⬇️
core/shared/src/main/scala/spire/math/Real.scala	67.19% <0%> (-0.32%)	⬇️
...re/shared/src/main/scala/spire/math/Rational.scala	96.02% <0%> (-0.25%)	⬇️
core/js/src/main/scala/spire/Platform.scala
...d/src/main/scala_2.13/spire/util/OptVersions.scala	66.66% <0%> (ø)
...red/src/main/scala/spire/math/poly/PolyDense.scala	97.36% <0%> (+1.31%)	⬆️
... and 3 more

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[tixxit]

LGTM