Negative exponents in SparsePolynomial

[jishnub]

Fixes #409, now

julia> SparsePolynomial(Dict(-1=>3, 2=>4))
SparsePolynomial(3*x^-1 + 4*x^2)

[codecov]

Codecov Report

Merging #411 (438143a) into master (8b1c45a) will increase coverage by 0.66%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #411      +/-   ##
==========================================
+ Coverage   83.11%   83.77%   +0.66%     
==========================================
  Files          23       23              
  Lines        2790     2799       +9     
==========================================
+ Hits         2319     2345      +26     
+ Misses        471      454      -17     

Impacted Files	Coverage Δ
src/common.jl	90.05% <100.00%> (+0.56%)	⬆️
src/polynomials/LaurentPolynomial.jl	95.33% <100.00%> (+5.54%)	⬆️
src/polynomials/SparsePolynomial.jl	100.00% <100.00%> (+1.70%)	⬆️
src/polynomials/standard-basis.jl	88.12% <100.00%> (+0.31%)	⬆️
src/polynomials/pi_n_polynomial.jl	77.77% <0.00%> (-22.23%)	⬇️
src/polynomials/factored_polynomial.jl	85.31% <0.00%> (+0.56%)	⬆️
src/show.jl	81.73% <0.00%> (+4.34%)	⬆️
... and 1 more

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[jverzani]

Looks nicely done. Thanks! I’ll merge in soon.

[jishnub]

This should be ready now

[jverzani]

I found an issue with integrate and derivative; let me merge this, then push those changes if they are still needed. Thanks so much for this!!