Add Base.round

[blegat]

See JuliaAlgebra/DynamicPolynomials.jl#4
CC @pb12345

[codecov-io]

Codecov Report

Merging #63 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   79.06%   79.15%   +0.08%     
==========================================
  Files          16       16              
  Lines         516      518       +2     
==========================================
+ Hits          408      410       +2     
  Misses        108      108

Impacted Files	Coverage Δ
src/polynomial.jl	77.9% <100%> (+0.52%)	⬆️

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[coveralls]

Coverage increased (+0.08%) to 79.151% when pulling 7a9bafc on round into d3836b5 on master.