Ngcd cleanup

[jverzani]

• rework numerical gcd code

[codecov]

Codecov Report

Merging #430 (c3d4df8) into master (76f3c9e) will decrease coverage by 0.81%.
The diff coverage is 80.20%.

@@            Coverage Diff             @@
##           master     #430      +/-   ##
==========================================
- Coverage   83.70%   82.88%   -0.82%     
==========================================
  Files          23       23              
  Lines        2798     2881      +83     
==========================================
+ Hits         2342     2388      +46     
- Misses        456      493      +37     

Impacted Files	Coverage Δ
src/polynomials/ngcd.jl	83.47% <78.82%> (-11.91%)	⬇️
src/polynomials/multroot.jl	92.79% <89.65%> (+0.41%)	⬆️
src/polynomials/pi_n_polynomial.jl	100.00% <100.00%> (+22.22%)	⬆️
src/polynomials/standard-basis.jl	89.15% <100.00%> (+0.90%)	⬆️
src/polynomials/ImmutablePolynomial.jl	91.66% <0.00%> (-1.67%)	⬇️
src/common.jl	90.75% <0.00%> (+1.12%)	⬆️

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