Compiler: print out sub_nodes if the a ast node contains Vec<AstNode>.

[WindSoilder]

After this pr #70, some ast nodes' like List are saved behind a Vec, which makes it hard to see what it actually contains, for example:

# list.nu
[1,2,3]

The compilder session outputs:

0: Int (11 to 12) "1"
1: Int (13 to 14) "2"
2: Int (15 to 16) "3"
3: List(ListId(0)) (10 to 16) 
4: Block(BlockId(0)) (0 to 18)

This pr is going to make it outputs one level detail:

0: Int (11 to 12) "1"
1: Int (13 to 14) "2"
2: Int (15 to 16) "3"
3: List(ListId(0)) (10 to 16) - sub_nodes: 0,1,2
4: Block(BlockId(0)) (0 to 18) - sub_nodes: 3

So it's easy to see that List contains node 0, 1, 2.

Avoid too verbosing

If one node contains too many(7) sub nodes, it replaces some nodes with ..., for example:

# list2.nu
[1,2,3,4,5,6,7,8]

It outputs:

0: Int (12 to 13) "1"
1: Int (14 to 15) "2"
2: Int (16 to 17) "3"
3: Int (18 to 19) "4"
4: Int (20 to 21) "5"
5: Int (22 to 23) "6"
6: Int (24 to 25) "7"
7: Int (26 to 27) "8"
8: List(ListId(0)) (11 to 27) - sub_nodes: 0,1,...,6,7
9: Block(BlockId(0)) (0 to 29) - sub_nodes: 8