add default constructors for iterators #76

Job	Run time
cache-clang	2m 30s
spelling	4s
build-ubuntu (mingw-ue3-x86-release)	30s
build-ubuntu (mingw-ue4-x64-release)	31s
build-ubuntu (clang-cross-ue3-x86-release)	17m 42s
build-ubuntu (clang-cross-ue4-x64-release)	16m 27s
clang-format	45s
clang-tidy (clang-ue3-x86-release)	8m 30s
build-windows (clang-ue3-x86-release)	2m 15s
clang-tidy (clang-ue4-x64-release)	8m 21s
build-windows (clang-ue4-x64-release)	2m 1s
build-windows (msvc-ue3-x86-release)	2m 5s
build-windows (msvc-ue4-x64-release)	3m 38s
	1h 5m 19s

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