Improve stdlib compatibility

[olegshaldybin]

1. Null values for primitive types no longer clear the original value in
   the destination object.

2. Dereference multiple levels of pointers in the destination interface{}
   type before unmarshaling into it. This is needed to match stdlib
   behavior when working with nested interface{} fields. If the destination
   object is a pointer to interface{} then the incoming nil value should
   nil out the destination object but keep the reference to that nil value
   on its parent object. However if the destination object is an
   interface{} value it should set the reference to nil but keep the
   original object intact.

3. Correctly handle typed nil decode destinations.

[codecov]

Codecov Report

Merging #172 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
- Coverage   84.89%   84.81%   -0.09%     
==========================================
  Files          41       41              
  Lines        5205     5196       -9     
==========================================
- Hits         4419     4407      -12     
- Misses        623      625       +2     
- Partials      163      164       +1

Impacted Files	Coverage Δ
feature_reflect_native.go	66.93% <100%> (-0.78%)	⬇️
feature_iter_object.go	67.12% <0%> (-3.43%)	⬇️
feature_iter_string.go	88.81% <0%> (-0.7%)	⬇️
feature_iter_skip.go	78.31% <0%> (+3.61%)	⬆️

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[taowen]

please fix the conflict

[olegshaldybin]

@taowen Done