elliptic-curve: remove `ToString` and `FromStr` impls #2774

Job	Run time
test (1.73.0)	27s
build (1.73.0, thumbv7em-none-eabi)	33s
test (stable)	28s
minimal-versions	0s
build (1.73.0, wasm32-unknown-unknown)	33s
test (nightly)	34s
build (stable, thumbv7em-none-eabi)	28s
build (stable, wasm32-unknown-unknown)	38s
	3m 41s

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