close issue 233 from polynomials

[jverzani]

• add method for Base.show_unquoted so that SymPy variables used with Polynomials have parentheses
• slight tweak to lambdify

[codecov-commenter]

Codecov Report

Merging #423 (0e4e510) into master (5a98def) will decrease coverage by 0.29%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
- Coverage   54.02%   53.73%   -0.30%     
==========================================
  Files          18       18              
  Lines        1155     1165      +10     
==========================================
+ Hits          624      626       +2     
- Misses        531      539       +8     

Impacted Files	Coverage Δ
src/lambdify.jl	93.22% <ø> (-0.12%)	⬇️
src/types.jl	35.41% <0.00%> (-5.06%)	⬇️
src/matrix.jl	71.62% <0.00%> (-0.85%)	⬇️

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