Add internal `_degeneratize_R` and `_degeneratize_I` #669

Job	Run time
Julia 1.6 - ubuntu-latest - x64 - pull_request	7m 55s
Julia 1.6 - macOS-latest - x64 - pull_request	5m 11s
Documentation	6m 14s
Julia 1.6 - windows-latest - x64 - pull_request	11m 39s
Julia 1 - ubuntu-latest - x64 - pull_request	3m 49s
Julia 1 - macOS-latest - x64 - pull_request	4m 54s
Julia 1 - windows-latest - x64 - pull_request	9m 23s
Julia nightly - ubuntu-latest - x64 - pull_request	7m 59s
Julia nightly - macOS-latest - x64 - pull_request	6m 23s
Julia nightly - windows-latest - x64 - pull_request	13m 7s
	1h 16m 34s

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