Implement crypto byte array newtypes in term of a shared type (#3684)

Introduces `CryptoBytes` type defined as:

```rust
pub struct CryptoBytes<const N: usize, Tag = ()>(pub [u8; N], PhantomData<fn() -> Tag>);
```

The type implements a bunch of methods and traits which are typically
expected from a byte array newtype
(NOTE: some of the methods and trait implementations IMO are a bit
redundant, but I decided to maintain them all to not change too much
stuff in this PR)

It also introduces two (generic) typical consumers of `CryptoBytes`:
`PublicBytes` and `SignatureBytes`.

```rust
pub struct PublicTag;
pub PublicBytes<const N: usize, CryptoTag> = CryptoBytes<N, (PublicTag, CryptoTag)>;

pub struct SignatureTag;
pub SignatureBytes<const N: usize, CryptoTag> = CryptoBytes<N, (SignatureTag, CryptoTag)>;
```

Both of them use a tag to differentiate the two types at a higher level.
Downstream specializations will further specialize using a dedicated
crypto tag. For example in ECDSA:


```rust
pub struct EcdsaTag;

pub type Public = PublicBytes<PUBLIC_KEY_SERIALIZED_SIZE, EcdsaTag>;
pub type Signature = PublicBytes<PUBLIC_KEY_SERIALIZED_SIZE, EcdsaTag>;
```

Overall we have a cleaner and most importantly **consistent** code for
all the types involved

All these details are opaque to the end user which can use `Public` and
`Signature` for the cryptos as before

polkadot/runtime/common/src/purchase.rs

@@ -425,8 +425,8 @@ pub mod pallet {
 impl<T: Config> Pallet<T> {
 	fn verify_signature(who: &T::AccountId, signature: &[u8]) -> Result<(), DispatchError> {
 		// sr25519 always expects a 64 byte signature.
-		let signature: AnySignature = sr25519::Signature::from_slice(signature)
-			.ok_or(Error::<T>::InvalidSignature)?
+		let signature: AnySignature = sr25519::Signature::try_from(signature)
+			.map_err(|_| Error::<T>::InvalidSignature)?
 			.into();
 
 		// In Polkadot, the AccountId is always the same as the 32 byte public key.