Refactoring ProtocolNumber as newtype

In the current implementation of network, the `ProtocolNumber` type, defined in `Network.Socket.Types`, is given as

```haskell
type ProtocolNumber = CInt
```

which means that it can only ever be displayed using the `show` method for `Foreign.C.Types.CInt`.

I initially was considering working on redefining it along the lines of

```haskell
newtype ProtocolNumber = ProtocolNumber CInt
deriving (Eq, Ord, Enum, Real, Num, Integral)
```

so that the implicit constraint `Num a => a` on numeric literals would be maintained, and so that it would be possible to define custom Read/Show instances that used pattern synonyms for commonly used protocols (e.g. TCP and UDP) for human-readability. (Along the lines of the pattern-synonym read/show work I implemented in #465 and #466)

Values that correspond to obsolete or extremely uncommon protocols could just be represented numerically, without any constructor syntax, as they are currently represented (and parsed) by default.

However, @vdukhovni pointed out that this would break the API for anyone using either variables or literals with explicit `:: CInt` signatures where `ProtocolNumber` values are expected, which could be either a trivial issue or a major problem depending on how common such usage is in the countless projects that have a dependency on network.

In any case, it seemed like enough of an obstacle that I wanted to consult with the project maintainers before I embark on a doomed mission that would break an indeterminate fraction of network-dependent code.