app_crypto_pair::from_seed(&Seed) should take ownership of seed

[gilescope]

app_crypto_pair::from_seed(_: &Seed)

to

app_crypto_pair::from_seed(_: Seed)

from_seed shouldn't be used so this doesn't really matter, but I think it's nicer if the function takes ownership of the seed.
(This PR looks even nicer once zebra lands as you can hand the ownership over to it)

(There's already a from_seed_slice if people really want to retain ownership)

[bkchr]

If from_seed doesn't require ownership of the seed, it shouldn't take it.

[gilescope]

Doesn't it feel more secure that it takes ownership of the seed rather than the seed continuing to float about memory until such time as it gets dropped? We could ensure at that point that the seed's memory was zeroed once we'd made use of it. (And if people did want to keep the seed we'd force them to clone it - this feels safer by design to me.)

[bkchr]

(And if people did want to keep the seed we'd force them to clone it - this feels safer by design to me.)

Why does this feels safer that the user needs to clone it? In the current design the user could ensure that the type is wrapped in a Zeroize wrapper. If the user would pass it, it would still not be automatically zeroed.