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

[AsterDY]

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-commenter]

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 ⚠️
ast/decode.go	0.00%	52 Missing ⚠️
ast/parser.go	66.01%	43 Missing and 9 partials ⚠️
ast/search.go	25.37%	44 Missing and 6 partials ⚠️
ast/api_amd64.go	57.44%	16 Missing and 4 partials ⚠️
api.go	14.28%	18 Missing ⚠️
internal/rt/fastmem.go	0.00%	4 Missing ⚠️
ast/visitor.go	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@            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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