Sparse vector polynomial

[jverzani]

• add a container type backed by a SparseVector
• add a new standard basis polynomial type SparseVectorPolynomial (not exported)
• identified a few issues (use _truncate! not truncate to avoid hints of piracy), issue with roots (length used, not degree)

[codecov]

Codecov Report

Patch coverage: 83.60% and project coverage change: +0.34% 🎉

Comparison is base (be37955) 75.16% compared to head (2b00b85) 75.51%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
+ Coverage   75.16%   75.51%   +0.34%     
==========================================
  Files          36       38       +2     
  Lines        3829     3998     +169     
==========================================
+ Hits         2878     3019     +141     
- Misses        951      979      +28     

Files Changed	Coverage Δ
src/Polynomials.jl	100.00% <ø> (ø)
src/polynomials/standard-basis/standard-basis.jl	70.84% <28.57%> (-0.74%)	⬇️
src/common.jl	80.15% <66.66%> (-0.21%)	⬇️
...ontainer-types/mutable-sparse-vector-polynomial.jl	81.25% <81.25%> (ø)
...nomials/standard-basis/sparse-vector-polynomial.jl	98.07% <98.07%> (ø)
...mial-container-types/immutable-dense-polynomial.jl	84.47% <100.00%> (ø)
...rc/polynomials/standard-basis/sparse-polynomial.jl	98.14% <100.00%> (ø)

... and 2 files with indirect coverage changes

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