add warning when duplicate hashmap key

[higuruchi]

Close: #174

Add warning when duplicate hashmap key

• yaml

  key1: 
    a: "b"
  key1: 
    b: "c"

• warning

  WARNING: Hash({String("b"): String("c")}) is duplicated
  

[puetzp]

Hey there, I wonder if insert_new_node should return a ScanError instead of a waning in case of encountering a duplicate key. It seems to me that the YAML 1.2 spec contains numerous sections that mention the requirement of key uniqueness in mappings, which would indicate a "hard" error.

[davvid]

👍 on the above, and also note that since this is library code we shouldn't really be printing anything.

I'd be more than happy to accept a PR that makes duplicate keys an error in my fork: https://github.com/davvid/yaml-rust