ast.rb: RubyVM::AST.parse and .of accepts save_script_lines: true

[mame]

Synopsis:

node = RubyVM::AbstractSyntaxTree.parse(<<END, save_script_lines: true)
puts 'Hello', 'world'
__END__
foobar
END

p node.script_lines #=> ["puts 'Hello', 'world'\n", "__END__\n"]

p node.source                                            #=> "puts 'Hello', 'world'"
p node.children.last.children.last.source                #=> "'Hello', 'world'"
p node.children.last.children.last.children.first.source #=> "'Hello'"

This option makes the parser keep the original source as an array of
the original code lines. This feature exploits the mechanism of
SCRIPT_LINES__ but records only the specified code that is passed to
RubyVM::AST.of or .parse, instead of recording all parsed program texts.

RubyVM::AST::Node instances created with save_script_lines: true
option respond to #script_lines and #source. The former returns
the whole original source code as an array of the lines. The latter returns
the code fragment that corresponds to this AST.
Note that RubyVM::AST is just for ruby internal use, so the compatibility
of the methods is never guaranteed.

This changeset is approved by @matz .