add unexported critical points

[jverzani]

An alternate means to address #454

Rather than specialize findmin etc. this utilizes an unexported critical_points function. With this, findmin etc. compose readily enough.

[codecov]

Codecov Report

Base: 83.23% // Head: 83.30% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (3735889) compared to base (efeb466).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 3735889 differs from pull request most recent head ad6cb53. Consider uploading reports for the commit ad6cb53 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   83.23%   83.30%   +0.06%     
==========================================
  Files          22       22              
  Lines        2994     3006      +12     
==========================================
+ Hits         2492     2504      +12     
  Misses        502      502              

Impacted Files	Coverage Δ
src/common.jl	90.12% <100.00%> (+0.26%)	⬆️
src/contrib.jl	82.47% <100.00%> (+0.36%)	⬆️

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[jverzani]

Maybe better to have this one function with a keyword argument:

function critical_points(p::AbstractPolynomial{T}, I=domain(p); endpoints=true) where {T <: Real}
    l, r = extrema(I)

    q = Polynomials.ngcd(derivative(p), derivative(p,2)).v

    pts = sort(real.(filter(isreal, roots(q))))
    !endpoints && return pts

    collect(Iterators.filter(x -> l ≤ x ≤ r,
                             Iterators.flatten((l, pts, r))))

end