Permit zero alloc reads

[lavalamp]

It's not currently possible to use jsoniter without having it allocate for each field name. This makes it possible. I found a couple existing functions that can benefit from this change, so it's overall a slight performance improvement.

benchmark                                         old ns/op     new ns/op     delta
Benchmark_encode_string_with_SetEscapeHTML-12     1373          1296          -5.61%
Benchmark_jsoniter_large_file-12                  25578         24599         -3.83%
Benchmark_json_large_file-12                      54500         54448         -0.10%
Benchmark_jsoniter_array-12                       161           162           +0.62%
Benchmark_json_array-12                           1722          1666          -3.25%
Benchmark_jsoniter_float-12                       27.8          26.1          -6.12%
Benchmark_json_float-12                           526           541           +2.85%
Benchmark_jsoniter_encode_int-12                  17.3          17.1          -1.16%
Benchmark_itoa-12                                 71.6          71.2          -0.56%
Benchmark_jsoniter_int-12                         20.8          22.0          +5.77%
Benchmark_json_int-12                             521           514           -1.34%
Benchmark_jsoniter_nested-12                      793           760           -4.16%
Benchmark_json_nested-12                          3032          2908          -4.09%
Benchmark_jsoniter_skip-12                        2075          2126          +2.46%
Benchmark_json_skip-12                            7361          7357          -0.05%

benchmark                                         old allocs     new allocs     delta
Benchmark_encode_string_with_SetEscapeHTML-12     6              6              +0.00%
Benchmark_jsoniter_large_file-12                  79             27             -65.82%
Benchmark_json_large_file-12                      14             14             +0.00%
Benchmark_jsoniter_array-12                       0              0              +0.00%
Benchmark_jsoniter_float-12                       0              0              +0.00%

benchmark                                         old bytes     new bytes     delta
Benchmark_encode_string_with_SetEscapeHTML-12     760           760           +0.00%
Benchmark_jsoniter_large_file-12                  4904          4576          -6.69%
Benchmark_json_large_file-12                      6632          6632          +0.00%
Benchmark_jsoniter_array-12                       0             0             +0.00%
Benchmark_jsoniter_float-12                       0             0             +0.00%

I added this benchmark to show the difference more clearly:

Benchmark_custom_scan/string-12                           100000             15149 ns/op            4816 B/op         76 allocs/op
Benchmark_custom_scan/bytes-12                            100000             12843 ns/op            4312 B/op          6 allocs/op

Often, users will store the field names and need to do this allocation anyway. But not always.

[codecov]

Codecov Report

Merging #394 into master will decrease coverage by 0.44%.
The diff coverage is 69.13%.

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
- Coverage   81.81%   81.37%   -0.45%     
==========================================
  Files          41       41              
  Lines        5022     5085      +63     
==========================================
+ Hits         4109     4138      +29     
- Misses        792      819      +27     
- Partials      121      128       +7

Impacted Files	Coverage Δ
iter_skip_strict.go	92.42% <100%> (ø)	⬆️
iter_str.go	85.71% <67.64%> (-5.2%)	⬇️
iter_object.go	63.08% <69.23%> (-3.4%)	⬇️
reflect_map.go	87.5% <71.42%> (ø)	⬆️
reflect_struct_decoder.go	48.74% <0%> (-0.42%)	⬇️

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[lavalamp]

Please let me know if this is something you're interested in, if so I can improve the coverage.