RakuAST: get meaningful .raku output for RakuAST classes

This should help with people writing RakuAST tests, and will
probably also be helpful in the future for people doing other stuff
with RakuAST, as you can now easily get the RakuAST calls for a
given piece of Raku source code.  Something like:

    say Q|sub Bar::foo($a) { say $a }|.AST.raku

now produces a roundtripping:

RakuAST::StatementList.new(
  RakuAST::Statement::Expression.new(
    expression => RakuAST::Sub.new(
      name      => RakuAST::Name.from-identifier-parts("Bar","foo")),
      signature => RakuAST::Signature.new(
        parameters => (
          RakuAST::Parameter.new(
            type   => RakuAST::Type::Simple.new(
              RakuAST::Name.from-identifier("Any")
            ),
            target => RakuAST::ParameterTarget::Var.new("\$a")
          ),
        )
      ),
      body      => RakuAST::Blockoid.new(
        RakuAST::StatementList.new(
          RakuAST::Statement::Expression.new(
            expression => RakuAST::Call::Name.new(
              name => RakuAST::Name.from-identifier("say"),
              args => RakuAST::ArgList.new(
                RakuAST::Var::Lexical.new("\$a")
              )
            )
          )
        )
      )
    )
  )
)