Support CharLiteral in RangeLiteral#each

[straight-shoota]

Currently, RangeLiteral#each and #map and #to_a as well as {% for range %} only work with numeric ranges. This patch adds support for ranges of CharLiteral, which we can iterate just like NumberLiteral.

Iterating CharLiteral is useful for building lookup tables for characters. They are better readable than the equivalent NumberLiteral expression.

For example:

NUMERIC_CHARS = {% begin %}
    {%
      table = (0...256).map { 0 }
      ('0'..'9').each do |c|
        table[c.ord] = 1
      end
    %}
    Slice.literal({{ table.splat }})
  {% end %}

Theoretically, we could do the same for StringLiteral, although I don't see a big use case for that.

The first two commits are a bit of prefactoring. I'll probably extract them to independent PRs.