Implement fixed-point differentiation and gauge-fixing #76

Job	Run time
Julia 1.6 - ubuntu-latest - x64	5m 28s
Julia 1.6 - macOS-latest - x64	6m 35s
Julia 1.6 - windows-latest - x64	18m 28s
Julia 1 - ubuntu-latest - x64	2m 15s
Julia 1 - macOS-latest - x64	3m 49s
Julia 1 - windows-latest - x64	3m 41s
Julia nightly - ubuntu-latest - x64	2m 26s
Julia nightly - macOS-latest - x64	2m 35s
Julia nightly - windows-latest - x64	3m 24s
	48m 41s

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