Export FakeFrame.Arg2KVIterator to faciliate tests

[cinchurge]

With arg2KVIterator unexported, it is extremely difficult for consumers to test with call frames when arg2 is involved. This change simply exports that attr so that is is possible to supply a custom iterator, such as via arg2.NewKeyValIterator.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2818e4c) 88.21% compared to head (878ca2f) 88.36%.
Report is 149 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #757      +/-   ##
==========================================
+ Coverage   88.21%   88.36%   +0.14%     
==========================================
  Files          40       40              
  Lines        4074     4074              
==========================================
+ Hits         3594     3600       +6     
+ Misses        364      361       -3     
+ Partials      116      113       -3     

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[devastating]

I think you need to export hasArg2KVIterator as well.

[prashantv]

For someone creating a fake call frame, does it make more sense to provide a setter that takes a map[string]string that it uses to create the iterator?

Similarly, should we merge Arg2StartOffset and Arg2EndOffset into a single Arg2Size? I think that's the only use-case we have for the values right?