`Astrolabe::Builder` is an AST builder that is utilized to let `Parser` generate AST with {Astrolabe::Node}.

@example

  require 'astrolabe/builder'
  require 'parser/current'

  buffer = Parser::Source::Buffer.new('(string)')
  buffer.source = 'puts :foo'

  builder = Astrolabe::Builder.new
  parser = Parser::CurrentRuby.new(builder)
  root_node = parser.parse(buffer)