don't use Val with ntuple when the length isn't a constant (#525)

Helps type inference.

Before:
```julia-repl
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-repl
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}}
```

Author: nsajko

src/polynomial-container-types/immutable-dense-polynomial.jl

@@ -98,7 +98,7 @@ function trim_trailing_zeros!!(cs::Tuple)
     !iszero(last(cs)) && return cs
     i = findlast(!iszero, cs)
     i == nothing && return ()
-    xs = ntuple(Base.Fix1(getindex,cs), Val(i))
+    xs = ntuple(Base.Fix1(getindex,cs), i)
     xs
 end
 
@@ -112,7 +112,7 @@ function Base.chop(p::ImmutableDensePolynomial{B,T,X,N};
         N′ = 0
     else
         N′ = i
-        xs = ntuple(Base.Fix1(getindex, p.coeffs), Val(N′))
+        xs = ntuple(Base.Fix1(getindex, p.coeffs), N′)
     end
     ImmutableDensePolynomial{B,T,X,N′}(xs)
 end