Added automated Cargo.toml upgrade CI (#7) #41

Job	Run time
Lint (ubuntu-latest, stable)	26s
Test (ubuntu-latest, default, stable)	44s
Lint (ubuntu-latest, beta)	21s
Test (ubuntu-latest, default, beta)	28s
Lint (ubuntu-latest, nightly)	17s
Test (ubuntu-latest, default, nightly)	23s
Lint (macos-latest, stable)	16s
Test (ubuntu-latest, serde_debugging, stable)	36s
Lint (macos-latest, beta)	24s
Test (ubuntu-latest, serde_debugging, beta)	28s
Lint (macos-latest, nightly)	18s
Test (ubuntu-latest, serde_debugging, nightly)	24s
Test (macos-latest, default, stable)	24s
Test (macos-latest, default, beta)	33s
Test (macos-latest, default, nightly)	42s
Test (macos-latest, serde_debugging, stable)	25s
Test (macos-latest, serde_debugging, beta)	33s
Test (macos-latest, serde_debugging, nightly)	31s
	8m 13s

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