Add PagedMergeSort, a merge sort using O(√n) space (#71) #146

Job	Run time
Julia 1.0 - ubuntu-latest - x64 - push	2m 26s
Julia 1.0 - ubuntu-latest - x86 - push	3m 23s
Julia 1.0 - macOS-latest - x64 - push	4m 39s
Julia 1.0 - windows-latest - x64 - push	8m 19s
Julia 1.0 - windows-latest - x86 - push	6m 10s
Julia 1 - ubuntu-latest - x64 - push	1m 35s
Julia 1 - ubuntu-latest - x86 - push	1m 53s
Julia 1 - macOS-latest - x64 - push	3m 4s
Julia 1 - windows-latest - x64 - push	1m 55s
Julia 1 - windows-latest - x86 - push	3m 24s
Julia nightly - ubuntu-latest - x64 - push	1m 28s
Julia nightly - ubuntu-latest - x86 - push	1m 37s
Julia nightly - macOS-latest - x64 - push	2m 26s
Julia nightly - windows-latest - x64 - push	4m 15s
Julia nightly - windows-latest - x86 - push	4m 42s
	51m 16s

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