Add polynomial composition

[mhostetter]

In [1]: import galois

In [2]: GF = galois.GF(31)

In [3]: f = galois.Poly([1, 0, 0, 15], field=GF); f
Out[3]: Poly(x^3 + 15, GF(31))

In [4]: g = galois.Poly.Degrees([3], field=GF); g
Out[4]: Poly(x^3, GF(31))

In [5]: f(g)
Out[5]: Poly(x^9 + 15, GF(31))

[codecov]

Codecov Report

Merging #377 (0e0a591) into master (371d7b1) will increase coverage by 0.00%.
The diff coverage is 93.90%.

@@           Coverage Diff           @@
##           master     #377   +/-   ##
=======================================
  Coverage   96.70%   96.70%           
=======================================
  Files          42       42           
  Lines        5213     5222    +9     
=======================================
+ Hits         5041     5050    +9     
  Misses        172      172           

Impacted Files	Coverage Δ
galois/_polys/_dense.py	97.83% <ø> (-0.08%)	⬇️
galois/_polys/_poly.py	96.43% <93.86%> (+0.08%)	⬆️
galois/_fields/_factory.py	96.52% <100.00%> (ø)

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