Fix iteration for SparsePolynomial

[jishnub]

This PR does two main things:

1. Change firstindex, lastindex and eachindex for SparsePolynomials to use the default fallback methods. This fixes Bug in sparse polynomial iteration #404.
2. Change PolynomialKeys and PolynomialValues to subtype AbstractSet, which, in the latter's case, involves adding a type-parameter. This makes empty SparsePolynomials work correctly by fetching the eltype from the parent type.

After this PR:

julia> p = SparsePolynomial([0])
SparsePolynomial(0)

julia> collect(p) # errors on master
Int64[]

julia> p = SparsePolynomial([0,1]) #404
SparsePolynomial(x)

julia> collect(p)
2-element Vector{Int64}:
 0
 1

Aside from this, there are some clean-ups of the iteration functions, an optimized length method for SparsePolynomial and fix some tests. Finally, change == checks for nothing to ===, which is the recommended style.

While the changes are largely bug-fixes, I've bumped the minor version as there are some technically breaking changes.

[codecov]

Codecov Report

Merging #405 (19eb252) into master (f6b7258) will decrease coverage by 0.13%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
- Coverage   83.16%   83.02%   -0.14%     
==========================================
  Files          23       23              
  Lines        2810     2793      -17     
==========================================
- Hits         2337     2319      -18     
- Misses        473      474       +1     

Impacted Files	Coverage Δ
src/common.jl	88.69% <88.88%> (-0.65%)	⬇️
src/polynomials/Poly.jl	82.43% <100.00%> (-0.91%)	⬇️
src/polynomials/SparsePolynomial.jl	98.29% <100.00%> (-0.03%)	⬇️

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[jverzani]

Wow. Thanks. I had gotten half way here this AM, but didn't think to fix the iterators so that pairs works as I had wanted. Most appreciated.