don't use Val with ntuple when the length isn't a constant

[nsajko]

Helps type inference.

Before:

julia> using Test, JET, Polynomials

julia> p = ImmutablePolynomial{Bool, :x, 1}((true,))
ImmutablePolynomial(true)

julia> @inferred coeffs(p)
ERROR: return type Tuple{Bool} does not match inferred return type Any

After:

julia> using Test, JET, Polynomials

julia> p = ImmutablePolynomial{Bool, :x, 1}((true,))
ImmutablePolynomial(true)

julia> @inferred coeffs(p)
ERROR: return type Tuple{Bool} does not match inferred return type Tuple{Vararg{Bool}}

[nsajko]

This also seems to wholly prevent run time dispatch for coeffs(::ImmutablePolynomial)! At least for small lengths? Not sure how this is possible.

Before:

julia> using Test, JET, Polynomials

julia> p = ImmutablePolynomial{Bool, :x, 7}((true,true,true,true,true,true,true))
ImmutablePolynomial(true + x + x^2 + x^3 + x^4 + x^5 + x^6)

julia> @report_opt coeffs(p)
═════ 1 possible error found ═════
┌ coeffs(p::ImmutablePolynomial{Bool, :x, 7}) @ Polynomials /home/nsajko/tmp/jl/Polynomials.jl/src/polynomial-container-types/immutable-dense-polynomial.jl:212
│┌ trim_trailing_zeros!!(cs::NTuple{7, Bool}) @ Polynomials /home/nsajko/tmp/jl/Polynomials.jl/src/polynomial-container-types/immutable-dense-polynomial.jl:101
││ runtime dispatch detected: Polynomials.ntuple(%92::Base.Fix1{typeof(getindex), NTuple{7, Bool}}, %94::Val)::Any
│└────────────────────

After:

julia> p = ImmutablePolynomial{Bool, :x, 7}((true,true,true,true,true,true,true))
ImmutablePolynomial(true + x + x^2 + x^3 + x^4 + x^5 + x^6)

julia> @report_opt coeffs(p)
No errors detected

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.28% 🎉

Comparison is base (5f5ba86) 75.31% compared to head (13ff12c) 76.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
+ Coverage   75.31%   76.59%   +1.28%     
==========================================
  Files          34       35       +1     
  Lines        3528     3739     +211     
==========================================
+ Hits         2657     2864     +207     
- Misses        871      875       +4     

Files Changed	Coverage Δ
...mial-container-types/immutable-dense-polynomial.jl	88.12% <100.00%> (+0.70%)	⬆️

... and 26 files with indirect coverage changes

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[jverzani]

Thanks again!!