fix #372: add AppendSkip iterator method

[ernado]

This PR adds AppendSkip method that will allow buffer reuse between skips, as described in #372

The following part was removed:

	if len(captured) == 0 {
		copied := make([]byte, len(remaining))
		copy(copied, remaining)
		return copied
	}

Because it is equivalent to append(captured, remaining...).

Also I've removed the unused captureBuffer structure.

Please ping me if you need any adjustments, clarification or additional tests.

[codecov]

Codecov Report

Merging #373 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #373      +/-   ##
=========================================
+ Coverage   81.57%   81.7%   +0.12%     
=========================================
  Files          41      41              
  Lines        5021    5022       +1     
=========================================
+ Hits         4096    4103       +7     
+ Misses        804     798       -6     
  Partials      121     121

Impacted Files	Coverage Δ
iter_skip.go	84.88% <100%> (+0.17%)	⬆️
reflect_struct_decoder.go	48.32% <0%> (+0.83%)	⬆️

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[taowen]

SkipAndReturnBytes and AppendSkip, the name should be similiar. Can you come up a better name?

[ernado]

Of course, what about SkipAndAppendBytes?