Tests

Improve `FieldElement` struct construction #1082

Job	Run time
no-std build	26s
Unix tests (ubuntu-latest, stable)	1m 45s
Unix tests (ubuntu-latest, nightly)	4m 3s
WASM tests	2m 37s
Unix tests (macos-11, stable)	14m 36s
Windows tests (stable, x86_64-pc-windows-gnu)	14m 44s
Unix tests (macos-11, nightly)	14m 36s
Windows tests (stable, x86_64-pc-windows-msvc)	7m 18s
Windows tests (nightly, x86_64-pc-windows-gnu)	10m 24s
Windows tests (nightly, x86_64-pc-windows-msvc)	14m 38s
	1h 25m 7s

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