add `SubDitStr` function to enable `DitStr` slicing (#54) #126

Job	Run time
Julia 1 - ubuntu-latest - x64 - push	2m 6s
Julia 1 - macOS-latest - x64 - push	1m 53s
Julia 1 - windows-latest - x64 - push	3m 17s
Julia nightly - ubuntu-latest - x64 - push	3m 34s
Julia nightly - macOS-latest - x64 - push	3m 38s
Julia nightly - windows-latest - x64 - push	5m 4s
	19m 32s

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