Add support for tagless enums.

[yallop]

It's quite common in C code to create enumerations without tags, so that they can only be referred to by an alias:

typedef enum {
  X,
  Y,
  Z
} xyz;

This PR adds support for exposing such types via an optional argument, typedef, to the enum function. The typedef flag indicates that the name passed to enum represents an alias, not an enumeration tag. With this PR the following code exposes an enueration like xyz above,

let i64 n = constant n int64_t
let t = enum "xyz" ~typedef:true [
  `X , i64 "X";
  `Y , i64 "Y";
  `Z , i64 "Z";
]

Omitting the ~typedef:true gives a binding suitable for an enumeration defined with a tag:

enum xyz {
  X,
  Y,
  Z
};