Add is_, as_, isreal, real, isintegerpoly & asintegerpoly

[singularitti]

Closes #258

[singularitti]

We can rename isintegerpoly & asintegerpoly to isintegral & asintegral, what do you think? The names come from "integral polynomials", an alias of "integer polynomials". It is said that "integral polynomials" were deprecated but I think isintegral is less ambiguous than "isinteger" here.

[jverzani]

isintegral is better than isintegerpoly.

[codecov]

Codecov Report

Merging #259 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
+ Coverage   87.46%   87.52%   +0.05%     
==========================================
  Files          15       15              
  Lines        1500     1507       +7     
==========================================
+ Hits         1312     1319       +7     
  Misses        188      188              

Impacted Files	Coverage Δ
src/common.jl	91.92% <100.00%> (+0.36%)	⬆️

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[jverzani]

Great and thanks! Once the tests go through, I'll get this merged in.