Eltype fix; use T[] for 0-polynomial; speed up LaurentPolynomial operations

[jverzani]

more generic

• In Remove buggy type computation in scalar_mult. #485 it was suggested to not guess output types of scalar multiplication. This moves more in that direction
• Uses T[] for the coefficients of a 0 polynomial, not T[0], which requires zero(T). This led to some fixes on evaluation and changed some tests
• new test in display of values which are 1 (hasone(T)) allows a few other polynomial constructions to be displayed

Speedups

• A comment in the LaurentPolynomials.jl package suggested performance problems with the LaurentPolynomial type. New approaches for * and + were added. Also, gcdx and powermod were borrowed from that package.
• The SparsePolynomial had a few arithmetic speedups, though it is still quite slow by comparison.
• fixed regression in scalar multiplication for the ImmutablePolynomial type

[codecov]

Codecov Report

Patch coverage: 78.50% and project coverage change: -2.62 ⚠️

Comparison is base (f5b20cd) 83.25% compared to head (c21f2af) 80.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
- Coverage   83.25%   80.64%   -2.62%     
==========================================
  Files          24       25       +1     
  Lines        3045     3151     +106     
==========================================
+ Hits         2535     2541       +6     
- Misses        510      610     +100     

Impacted Files	Coverage Δ
src/abstract.jl	51.02% <ø> (ø)
src/polynomials/pi_n_polynomial.jl	77.77% <14.28%> (-22.23%)	⬇️
src/polynomials/standard-basis.jl	80.10% <40.00%> (-9.14%)	⬇️
src/polynomials/factored_polynomial.jl	86.45% <50.00%> (-0.39%)	⬇️
src/contrib.jl	79.61% <66.66%> (-2.87%)	⬇️
src/show.jl	80.83% <66.66%> (-0.53%)	⬇️
src/polynomials/SparsePolynomial.jl	91.20% <91.42%> (-8.80%)	⬇️
src/polynomials/Polynomial.jl	97.56% <94.11%> (-2.44%)	⬇️
src/common.jl	87.21% <100.00%> (-2.71%)	⬇️
src/polynomials/ImmutablePolynomial.jl	91.59% <100.00%> (-0.28%)	⬇️
... and 2 more

... and 1 file with indirect coverage changes

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