json2: new decoder2 part 1

[enghitalo]

This is a part of json2 decoder2 rewriting in https://github.com/enghitalo/v/tree/json2/decode2-precalc-and-validation
It was splitted to help in review

This fraction is related to:

• new json cheking
• v type checking
• new decoding strategy using ValueInfo

OBS: I am thinking about split the already done work in around 5 parts. What not means this work is done

benchmark keeps great

./v -prod crun vlib/x/json2/decoder2/tests/bench.v                          ─╯
 SPENT    35.895 ms in old_json.decode(Stru, json_data)!

 SPENT    12.861 ms in decoder2.decode[StructType[string]](json_data1)!
 SPENT     9.902 ms in old_json.decode(StructType[string], json_data1)!

 SPENT    13.673 ms in decoder2.decode[StructTypeOption[string]](json_data1)!
 SPENT    11.536 ms in old_json.decode(StructTypeOption[string], json_data1)!

 SPENT    15.311 ms in decoder2.decode[map[string]string](json_data1)!
 SPENT    29.128 ms in old_json.decode(map[string]string, json_data1)!