Add internal `_degeneratize_R` and `_degeneratize_I` #668

Job	Run time
Julia 1.6 - ubuntu-latest - x64 - pull_request	5m 20s
Documentation	6m 48s
Julia 1.6 - macOS-latest - x64 - pull_request	5m 51s
Julia 1.6 - windows-latest - x64 - pull_request	18m 26s
Julia 1 - ubuntu-latest - x64 - pull_request	4m 57s
Julia 1 - macOS-latest - x64 - pull_request	7m 16s
Julia 1 - windows-latest - x64 - pull_request	8m 36s
Julia nightly - ubuntu-latest - x64 - pull_request	16m 59s
Julia nightly - macOS-latest - x64 - pull_request	9m 19s
Julia nightly - windows-latest - x64 - pull_request	10m 50s
	1h 34m 22s

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