feature #384: Specify custom map key sort function

[zachelrath]

Addresses #384 , adding the ability to specify a custom map key sort function via a registered extension. Defaults to the previous behavior, of just sorting map keys alphanumerically using < operator

Example use:

 type prioritizedSorter struct { prioritizedKey string } func (sorter *prioritizedSorter) Sort(keyA string, keyB string) bool {	// Prioritize our configured key, otherwise alpha-sort	if keyA == prioritizedKey {	return true	} else if keyB == prioritizedKey {	return false	} else {	return keyA < keyB	} } type prioritizedSortExtension struct { jsoniter.DummyExtension sorter prioritizedSorter } func (extension *prioritizedSortExtension) CreateMapKeySorter() jsoniter.MapKeySorter {	return extension.sorter } ... cfg := jsoniter.Config{ SortMapKeys: true, }.Froze() cfg.RegisterExtension(&prioritizedSortExtension{ sorter: &prioritizedSorter{ prioritizedKey: "trump", }, }) m := map[string]int{ "b": "sdfasdf" "trump": "i should be first", "a": "asdfasf", } output, _ := cfg.MarshalToString(m) 

[codecov]

Codecov Report

Merging #385 into master will increase coverage by 0.12%.
The diff coverage is 83.33%.

@@ Coverage Diff @@ ## master #385 +/- ## ========================================== + Coverage 81.64% 81.76% +0.12%  ========================================== Files 41 41 Lines 5022 5041 +19 ========================================== + Hits 4100 4122 +22  + Misses 801 798 -3  Partials 121 121

Impacted Files	Coverage Δ
reflect.go	92.7% <ø> (ø)	⬆️
reflect_map.go	88.26% <100%> (+0.76%)	⬆️
reflect_extension.go	83.4% <33.33%> (+0.41%)	⬆️
reflect_struct_decoder.go	48.32% <0%> (+0.41%)	⬆️

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