fix: validate on demand at first for GetByPath and NewRaw

[liuq19]

To make the ast API safer, compatible with sonic early versions(< v1.7.0), and remain the optimization by SIMD skip.
We make sure that the raw jsons in ast.Node are always well-formed and valid as possible.

Main Changes:

1. GetByPath: always validate the ondemand fields at first.
2. GetByPath: check path, panic when the path is not string or integer(>=0)
3. NewRaw: validate the input json, make sure all nodes' json is well-formed and valid.

goos: linux
goarch: amd64
pkg: github.com/bytedance/sonic/ast
cpu: Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz
                │ /tmp/tmplsy6pra1.source.txt │      /tmp/tmpnyhtp7f1.target.txt       │
                │           sec/op            │    sec/op     vs base                  │
GetFull_Sonic-8                   1.506µ ± 7%   24.492µ ± 3%  +1526.26% (p=0.000 n=10)
GetOne_Sonic-8                    2.522µ ± 8%    2.506µ ± 5%          ~ (p=0.579 n=10)
geomean                           1.949µ         7.834µ        +301.95%

                │ /tmp/tmplsy6pra1.source.txt │     /tmp/tmpnyhtp7f1.target.txt      │
                │             B/s             │     B/s       vs base                │
GetFull_Sonic-8                 8245.1Mi ± 6%   507.1Mi ± 3%  -93.85% (p=0.000 n=10)
GetOne_Sonic-8                   4.808Gi ± 7%   4.840Gi ± 5%        ~ (p=0.579 n=10)
geomean                          6.222Gi        1.548Gi       -75.12%

                │ /tmp/tmplsy6pra1.source.txt │     /tmp/tmpnyhtp7f1.target.txt     │
                │            B/op             │    B/op      vs base                │
GetFull_Sonic-8                   0.00 ± 0%     19.50 ± 18%        ? (p=0.000 n=10)
GetOne_Sonic-8                   24.00 ± 0%     46.50 ±  5%  +93.75% (p=0.000 n=10)
geomean                                     ¹   30.11        ?
¹ summaries must be >0 to compute geomean

                │ /tmp/tmplsy6pra1.source.txt │     /tmp/tmpnyhtp7f1.target.txt     │
                │          allocs/op          │ allocs/op   vs base                 │
GetFull_Sonic-8                  0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
GetOne_Sonic-8                   1.000 ± 0%     1.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                     ²               +0.00%