Remove `From` impls resulting in silent field element conversions #917

Job	Run time
Clippy	40s
no-std (stable, wasm32-unknown-unknown)	32s
rustfmt	25s
Check with --all-features	30s
no-std (nightly, wasm32-unknown-unknown)	39s
Test Rust stable on ubuntu	1m 20s
Check with --no-default-features	30s
Test Rust nightly on ubuntu	1m 13s
	5m 49s

Provide feedback