feat: introduce `ast.Value` to support read and write concurrently

Background

sonic/ast.Node provide completed and efficient APIs to manipulate JSON. However, due to its transversely-lazy-load design, it CANNOT be read concurrently, result in limited usage in practice -- or abuse usage of LoadAll(). Therefore, we consider introduce a new type ast.Value, whose APIs are all based on JSON skipping-and-searching algorithm, can naturally support concurrently read

Thread Safty

GetXX(): All read APIs are implemented by searching-and-skipping raw JSON, and returned a slice of the origin -- Thus it is thread-safe
SetXX(): All write APIs are implemented based on GetXX(), and returned a new copy to the origin -- Thus it is also thread-safe, while the correctness of currently-writing need user to ensure by itself

Performance

Value's performance is close to Node, and 3x~10x times faster than the same API of gjson|sjson!!!

codes are here

goos: darwin
goarch: amd64
pkg: github.com/bytedance/sonic/external_jsonlib_test/benchmark_test
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkGet/gjson-16      24603         43020 ns/op           1 B/op          0 allocs/op
BenchmarkGet/Node-16      289396          4206 ns/op         333 B/op          1 allocs/op
BenchmarkGet/Value-16     298384          4442 ns/op         342 B/op          1 allocs/op
BenchmarkSubGet/gjson-16               10000        107682 ns/op           2 B/op          0 allocs/op
BenchmarkSubGet/Node-16                20682         60565 ns/op        5976 B/op          5 allocs/op
BenchmarkSubGet/Value-16               19473         59041 ns/op        1563 B/op          0 allocs/op
BenchmarkGetMany/gjson-16               7738        155706 ns/op         242 B/op          1 allocs/op
BenchmarkGetMany/Node-16               22348         51654 ns/op        3697 B/op          5 allocs/op
BenchmarkGetMany/Value-16              26848         49827 ns/op        1493 B/op          0 allocs/op
BenchmarkSetMany/sjson-16               4611        253986 ns/op       58056 B/op         18 allocs/op
BenchmarkSetMany/Node-16               24410         50759 ns/op        3096 B/op          6 allocs/op
BenchmarkSetMany/Value-16              16446         72621 ns/op       70596 B/op         12 allocs/op
BenchmarkSet/sjson-16                   7494        179970 ns/op       53008 B/op         11 allocs/op
BenchmarkSet/Node-16                   42609         28077 ns/op        2704 B/op          6 allocs/op
BenchmarkSet/Value-16                  60609         17365 ns/op       29228 B/op          5 allocs/op

Codecov Report

Attention: Patch coverage is 58.84808% with 493 lines in your changes are missing coverage. Please review.

Project coverage is 76.63%. Comparing base (3739ffe) to head (bbf63a6).

Files	Patch %	Lines
ast/raw.go	64.67%	194 Missing and 102 partials :warning:
ast/decode.go	0.00%	52 Missing :warning:
ast/parser.go	66.01%	43 Missing and 9 partials :warning:
ast/search.go	25.37%	44 Missing and 6 partials :warning:
ast/api_amd64.go	57.44%	16 Missing and 4 partials :warning:
api.go	14.28%	18 Missing :warning:
internal/rt/fastmem.go	0.00%	4 Missing :warning:
ast/visitor.go	50.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #579 +/- ## ========================================== - Coverage 77.56% 76.63% -0.93% ========================================== Files 83 84 +1 Lines 11542 12581 +1039 ========================================== + Hits 8952 9642 +690 - Misses 2192 2408 +216 - Partials 398 531 +133 ```

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bytedance / sonic