Remove intermediary reader and directly feed iterator for unmarshal

[markusthoemmes]

When not streaming the data, we might as well just use jsoniter's pools as intended and feed the iterator with the ready-to-go byte slice directly.
Kept the reader interface intact for enhancements on the streaming side.

Signed-off-by: Markus Thömmes markusthoemmes@me.com

Benchmark

benchmark                                                                                       old ns/op     new ns/op     delta
BenchmarkUnmarshal/struct_data_v0.3-16                                                          17204         16991         -1.24%
BenchmarkUnmarshal/data,_attributes_and_extensions_and_specversion_with_struct_data_v0.3-16     17956         18042         +0.48%
BenchmarkUnmarshal/data,_attributes_and_extensions_and_specversion_with_struct_data_v1.0-16     18128         17969         -0.88%
BenchmarkUnmarshal/string_data_v1.0-16                                                          16832         16925         +0.55%
BenchmarkUnmarshal/nil_data_v1.0-16                                                             16021         16136         +0.72%
BenchmarkUnmarshal/string_data_v0.3-16                                                          16729         16958         +1.37%
BenchmarkUnmarshal/nil_data_v0.3-16                                                             15394         15561         +1.08%
BenchmarkUnmarshal/struct_data_v1.0-16                                                          17052         17108         +0.33%
BenchmarkUnmarshal/base64_json_encoded_data_v1.0-16                                             11311         11341         +0.27%
BenchmarkUnmarshal/base64_xml_encoded_data_v1.0-16                                              12085         11786         -2.47%

benchmark                                                                                       old allocs     new allocs     delta
BenchmarkUnmarshal/struct_data_v0.3-16                                                          76             75             -1.32%
BenchmarkUnmarshal/data,_attributes_and_extensions_and_specversion_with_struct_data_v0.3-16     83             82             -1.20%
BenchmarkUnmarshal/data,_attributes_and_extensions_and_specversion_with_struct_data_v1.0-16     83             82             -1.20%
BenchmarkUnmarshal/string_data_v1.0-16                                                          73             72             -1.37%
BenchmarkUnmarshal/nil_data_v1.0-16                                                             71             70             -1.41%
BenchmarkUnmarshal/string_data_v0.3-16                                                          75             74             -1.33%
BenchmarkUnmarshal/nil_data_v0.3-16                                                             71             70             -1.41%
BenchmarkUnmarshal/struct_data_v1.0-16                                                          74             73             -1.35%
BenchmarkUnmarshal/base64_json_encoded_data_v1.0-16                                             49             48             -2.04%
BenchmarkUnmarshal/base64_xml_encoded_data_v1.0-16                                              49             48             -2.04%

benchmark                                                                                       old bytes     new bytes     delta
BenchmarkUnmarshal/struct_data_v0.3-16                                                          3584          3533          -1.42%
BenchmarkUnmarshal/data,_attributes_and_extensions_and_specversion_with_struct_data_v0.3-16     3739          3688          -1.36%
BenchmarkUnmarshal/data,_attributes_and_extensions_and_specversion_with_struct_data_v1.0-16     3739          3688          -1.36%
BenchmarkUnmarshal/string_data_v1.0-16                                                          3392          3340          -1.53%
BenchmarkUnmarshal/nil_data_v1.0-16                                                             3351          3301          -1.49%
BenchmarkUnmarshal/string_data_v0.3-16                                                          3568          3517          -1.43%
BenchmarkUnmarshal/nil_data_v0.3-16                                                             3352          3300          -1.55%
BenchmarkUnmarshal/struct_data_v1.0-16                                                          3407          3357          -1.47%
BenchmarkUnmarshal/base64_json_encoded_data_v1.0-16                                             2708          2659          -1.81%
BenchmarkUnmarshal/base64_xml_encoded_data_v1.0-16                                              2724          2675          -1.80%

Not a huge difference at all but 🤷‍♂️. Seemed worth to use the lib as inteded (with the non-buffered iterator pool)