Add `TryFrom<u64>` and `TryFrom<u128>` bounds to the `FieldElement` trait #923

Job	Run time
Clippy	23s
rustfmt	16s
no-std (stable, wasm32-unknown-unknown)	34s
Test Rust stable on ubuntu	1m 13s
no-std (nightly, wasm32-unknown-unknown)	38s
Check with --all-features	22s
Test Rust nightly on ubuntu	1m 20s
Check with --no-default-features	32s
	5m 18s

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