Copy slice instead of iterating in wrap()

[mriise]

digest.copy_from_slice(input_digest);
instead of

rust-multihash/src/multihash.rs

Lines 107 to 111 in c68638c

	let mut i = 0;
	while i < size {
	digest[i] = input_digest[i];
	i += 1;
	}

This will stay open till rust-lang/rust#57349

[BigLep]

@mriise : will you take this?

[vmx]

@BigLep: this issue will likely stay open until the linked Rust issue if fixed (can be years).

[mriise]

(though we dont actually use lables for this repo lol)