Make SExponents opaque isbits type

[saschatimme]

Currently SExponents has a NTuple{N, Int} in its type parameter. I noticed that for large tuples this can trip up inference. This PR changes SExponents to be a isbits type of the form

struct SExponents{N}
    exponents::NTuple{N, UInt8}
    size::Tuple{Int,Int} # nvars, nterms
end

and Polynomial now takes an instance of SExponents instead of a type.

[codecov-io]

Codecov Report

Merging #12 into master will decrease coverage by 0.16%. The diff coverage is 73.68%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage   88.13%   87.96%   -0.17%     
==========================================
  Files          10       10              
  Lines         455      457       +2     
==========================================
+ Hits          401      402       +1     
- Misses         54       55       +1

Impacted Files	Coverage Δ
src/system.jl	51.94% <0%> (ø)	:arrow_up:
src/show.jl	91.25% <100%> (ø)	:arrow_up:
src/evaluation.jl	92.59% <100%> (ø)	:arrow_up:
src/polynomial.jl	96.55% <100%> (ø)	:arrow_up:
src/sexponents.jl	71.42% <63.63%> (-3.58%)	:arrow_down:

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