Support for infinite terms

[HarrisonGrodin]

Rewrite should be able to store and normalize infinite terms. The following forms seem especially important, although the implementation of infinite terms overall should ideally be generalized to fit these and more.

• Infinite sums and products
• Infinite nested radicals
• Infinite power towers
• Infinite continued fraction

julia> t = Sum(@term(k), @term((-1)^k / factorial(2k + 1) * x^(2k + 1)), 0, Inf);

julia> normalize(@term(diff($t, x)))
Sum(@term(k), @term((-1)^k / factorial(2k) * x^(2k)), 0, Inf)