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/primitives/test-helpers/src/lib.rs

@@ -136,17 +136,17 @@ pub fn dummy_head_data() -> HeadData {
 
 /// Create a meaningless collator id.
 pub fn dummy_collator() -> CollatorId {
-	CollatorId::from(sr25519::Public::from_raw([0; 32]))
+	CollatorId::from(sr25519::Public::default())
 }
 
 /// Create a meaningless validator id.
 pub fn dummy_validator() -> ValidatorId {
-	ValidatorId::from(sr25519::Public::from_raw([0; 32]))
+	ValidatorId::from(sr25519::Public::default())
 }
 
 /// Create a meaningless collator signature.
 pub fn dummy_collator_signature() -> CollatorSignature {
-	CollatorSignature::from(sr25519::Signature([0u8; 64]))
+	CollatorSignature::from(sr25519::Signature::default())
 }
 
 /// Create a meaningless persisted validation data.