Allow negative exponents for GroupElement.exp #2402

Job	Run time
Test JNI(Android) bindings	12m 16s
Clippy (linter)	1m 9s
Check intra-documentation links	6m 16s
Build without default features	1m 21s
Test Swift(iOS) bindings	3m 16s
Run JS tests and publish alpha versions	22m 4s
Code formatting (rustfmt)	12s
Tests on ubuntu-latest	5m 21s
Code coverage in tests	24m 36s
Tests on windows-latest	11m 24s
Tests on macOS-latest	4m 35s
	1h 32m 30s

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