In the ancient Ruby, symbols are represented by integers. In that era,
to get string representation from integers, we used `Integer#id2sym`
method. Later, `Symbol` was introduced, and `id2sym` was used for
compatibility. Today, no one uses `id2sym` any longer. It is described
in ISO 30170:2012 standard but I consider it as a mistake.
Even though `Symbol#to_proc` is not included in ISO standard, the
`some_method(&:method_name)` is used very widely and convenient.
So we moved it to the core.
Yukihiro "Matz" Matsumoto