Use private constants to define :toc_levels option

[ashmaroli]

Motivation

This is to reduce array allocation when multiple Kramdown::Document objects are initialized with the same :toc_levels value.

Summary

When Kramdown::Document is initialized with option :toc_levels => [1, 2, 3, 4, 5, 6] use a predefined constant with identical contents instead of generating a new array from the option value or when Kramdown::Document is initialized with option :toc_levels => (1..6), use a predefined constant array instead of converting the range into a new Array.

It is safe to use constant values because the (merged) options attribute of Kramdown::Document instances is already frozen on initialization.

[gettalong]

Thank you!