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[bot]]

Codecov Report

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

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

```diff @@ 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](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath) | Coverage Δ | | |---|---|---| | [src/Polynomials.jl](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL1BvbHlub21pYWxzLmps) | `100.00% <ø> (ø)` | | | [src/polynomials/standard-basis/standard-basis.jl](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL3BvbHlub21pYWxzL3N0YW5kYXJkLWJhc2lzL3N0YW5kYXJkLWJhc2lzLmps) | `70.84% <28.57%> (-0.74%)` | :arrow_down: | | [src/common.jl](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL2NvbW1vbi5qbA==) | `80.15% <66.66%> (-0.21%)` | :arrow_down: | | [...ontainer-types/mutable-sparse-vector-polynomial.jl](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL3BvbHlub21pYWwtY29udGFpbmVyLXR5cGVzL211dGFibGUtc3BhcnNlLXZlY3Rvci1wb2x5bm9taWFsLmps) | `81.25% <81.25%> (ø)` | | | [...nomials/standard-basis/sparse-vector-polynomial.jl](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL3BvbHlub21pYWxzL3N0YW5kYXJkLWJhc2lzL3NwYXJzZS12ZWN0b3ItcG9seW5vbWlhbC5qbA==) | `98.07% <98.07%> (ø)` | | | [...mial-container-types/immutable-dense-polynomial.jl](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL3BvbHlub21pYWwtY29udGFpbmVyLXR5cGVzL2ltbXV0YWJsZS1kZW5zZS1wb2x5bm9taWFsLmps) | `84.47% <100.00%> (ø)` | | | [...rc/polynomials/standard-basis/sparse-polynomial.jl](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath#diff-c3JjL3BvbHlub21pYWxzL3N0YW5kYXJkLWJhc2lzL3NwYXJzZS1wb2x5bm9taWFsLmps) | `98.14% <100.00%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/JuliaMath/Polynomials.jl/pull/536/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaMath)

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