Add ValidParser

[kevburnsjr]

Presently, users that want to validate their json before parsing need to run Validate and Parse separately.
This is inefficient because each json string must be scanned twice.

This PR introduces a ValidParser which validates as it parses.

It is slightly less efficient than the normal parser, but much more efficient than running Validate and Parse separately.

BenchmarkParse/small/fastjson-12                        10067671      111.0 ns/op  1712.14 MB/s    0 B/op  0 allocs/op
BenchmarkParse/small/fastjson-valid-parser-12           10503465      109.8 ns/op  1729.86 MB/s    0 B/op  0 allocs/op
BenchmarkParse/small/fastjson-validate-and-parse-12      6553486      176.5 ns/op  1076.23 MB/s    0 B/op  0 allocs/op

BenchmarkParse/medium/fastjson-12                        1797662      639.2 ns/op  3643.68 MB/s    0 B/op  0 allocs/op
BenchmarkParse/medium/fastjson-valid-parser-12           1635453      795.7 ns/op  2927.14 MB/s    0 B/op  0 allocs/op
BenchmarkParse/medium/fastjson-validate-and-parse-12      945916     1400 ns/op    1663.97 MB/s    0 B/op  0 allocs/op

BenchmarkParse/large/fastjson-12                          121093     9001 ns/op    3123.79 MB/s    0 B/op  0 allocs/op
BenchmarkParse/large/fastjson-valid-parser-12             116598    10946 ns/op    2568.69 MB/s    0 B/op  0 allocs/op
BenchmarkParse/large/fastjson-validate-and-parse-12        57669    20630 ns/op    1362.95 MB/s    0 B/op  0 allocs/op

BenchmarkParse/canada/fastjson-12                           1072  1205458 ns/op    1867.39 MB/s    1 B/op  0 allocs/op
BenchmarkParse/canada/fastjson-valid-parser-12              1022  1151216 ns/op    1955.38 MB/s    1 B/op  0 allocs/op
BenchmarkParse/canada/fastjson-validate-and-parse-12         678  1781026 ns/op    1263.91 MB/s    2 B/op  0 allocs/op

BenchmarkParse/citm/fastjson-12                             2584   476059 ns/op    3628.13 MB/s    0 B/op  0 allocs/op
BenchmarkParse/citm/fastjson-valid-parser-12                2528   481104 ns/op    3590.09 MB/s    0 B/op  0 allocs/op
BenchmarkParse/citm/fastjson-validate-and-parse-12          1443   844153 ns/op    2046.08 MB/s    1 B/op  0 allocs/op

BenchmarkParse/twitter/fastjson-12                          6711   164653 ns/op    3835.42 MB/s  756 B/op  0 allocs/op
BenchmarkParse/twitter/fastjson-valid-parser-12             5918   192253 ns/op    3284.80 MB/s    0 B/op  0 allocs/op
BenchmarkParse/twitter/fastjson-validate-and-parse-12       3758   317174 ns/op    1991.06 MB/s    0 B/op  0 allocs/op

Adds 7 new symbols to the API:

type ValidScanner
type ValidParser 
type ValidParserPool
func ValidParse(s string) (*Value, error)
func ValidParseBytes(b []byte) (*Value, error)
func MustValidParse(s string) *Value
func MustValidParseBytes(b []byte) *Value

See #58

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6dae91c) 97.11% compared to head (80b9537) 97.77%. Report is 2 commits behind head on master.

:exclamation: Current head 80b9537 differs from pull request most recent head f240cbf. Consider uploading reports for the commit f240cbf to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #68 +/- ## ========================================== + Coverage 97.11% 97.77% +0.66% ========================================== Files 9 11 +2 Lines 1073 1306 +233 ========================================== + Hits 1042 1277 +235 + Misses 22 21 -1 + Partials 9 8 -1 ```

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[flowchartsman]

I, too, do not enjoy needing two calls for peace-of-mind that my JSON will not break once I start processing it. This would be super handy to have merged into the library, and the PR looks well-tested and benchmarked.

@valyala, is there some hesitation with this feature which has left it in limbo for so long? If not, I think a lot of people would appreciate being able to avoid separate invocations. 🙏

[kevburnsjr]

I've updated the symbol names. ValidateParser is now ValidParser for brevity. I will maintain a fork with this PR merged until this PR is merged for anyone who wants to use it. https://github.com/logbn/fastjson-valid