Add `TryFrom<u64>` and `TryFrom<u128>` bounds to the `FieldElement` trait #925

Job	Run time
no-std (stable, wasm32-unknown-unknown)	36s
Check with --all-features	23s
no-std (nightly, wasm32-unknown-unknown)	41s
Check with --no-default-features	21s
rustfmt	13s
Clippy	22s
Test Rust stable on ubuntu	1m 9s
Test Rust nightly on ubuntu	1m 13s
	4m 58s

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