Added ‘char’ (c) and ‘asInt’ instances formatting Enum

Simple character formatter and a formatter that formats values as an `Int` using `fromEnum`:

```haskell
ghci> mapM_ TIO.putStrLn [ format (char % " has value " % asInt) ch ch | ch <- "abc" ]
a has value 97
b has value 98
c has value 99
```

As a vague motivator the Rust tutorial has an [examples](http://doc.rust-lang.org/tutorial.html#enums) of formatting using what is effectively `fromEnum`:

```rust
println!( "North => {}", North as int );
```

which would be:

```haskell
ghci> data Direction = South | North | East | West deriving Enum
ghci> format ("North => " % asInt) North
"North => 1"
```