Support for enums

Can use an enum array, which stores the index:

```coffee
level = bison.enumeration 'uint8', [ 'admin', 'reader', 'writer' ]

types = bison.preCompile
  'level': level
```

or an enum object, which stores the given value:

```coffee
level = bison.enumeration 'uint16',
  admin: 0x7c98
  reader: 0xb12d
  writer: 0x9f3a

types = bison.preCompile
  'level': level
```