AST::List: Make List constructor flatten sublists

Makes this:

$ perl author_tools/jit_ast_dump.pl -c -e 'my @a; my $x; @a = @x[2,@a,(2,(@a)), 1]'
----------------------------------------------
 Binop 'array slice' (
   List(
     C = (IV)2
     V (@) = 1 : Array[Any]
     List(
       C = (IV)2
       V (@) = 1 : Array[Any]
     )
     C = (IV)1
   )
   G = (@)
 )

turn into this:
 Binop 'array slice' (
   List(
     C = (IV)2
     V (@) = 1 : Array[Any]
     C = (IV)2
     V (@) = 1 : Array[Any]
     C = (IV)1
   )
   G = (@)
 )
AST: s///
AST: First stab at OP_REPEAT