Method invalidation on loading Polynomials

```julia
julia> using SnoopCompileCore

julia> invalidations = @snoopr using Polynomials;

julia> using SnoopCompile

julia> trees = invalidation_trees(invalidations);

julia> length(trees)
6

julia> trees[end].method
convert(::Type{T}, p::P) where {T, P<:(AbstractPolynomial{T})} in Polynomials at /home/jishnu/Dropbox/JuliaPackages/Polynomials.jl/src/common.jl:433

julia> trees[end-1].method
promote_rule(::Type{<:SparsePolynomial{var"#336#T", var"#338#X"}}, ::Type{var"#337#S"}) where {var"#336#T", var"#337#S"<:Number, var"#338#X"} in Polynomials at /home/jishnu/Dropbox/JuliaPackages/Polynomials.jl/src/abstract.jl:90

julia> trees[end-3].method
any(pred, p::AbstractPolynomial) in Polynomials at /home/jishnu/Dropbox/JuliaPackages/Polynomials.jl/src/common.jl:493
```

These three methods introduced here cause a lot of invalidation, leading to slow package load times. Maybe we can think about ways in which this may be remedied.