Token locations

[soutaro]

This PR is to store token-level locations in AST locations. It introduces Location::WithChildren class which saves child locations inside a location.

loc = Location::WithChildren.new(buffer: buffer, start_pos: 0, end_pos: 13)
loc = loc.merge_required({ name: 1...5 })
loc = loc.merge_optional({ args: 5...13 })

loc[:name]      # => Location instance for `Array`
loc[:args]      # => Location instance for `[String]`

AST now stores locations for its name tokens and some of the keyword tokens.