Allow explicit enum values

[harrisi]

This PR adds the ability to set values for enums, so this type of C code can be generated:

enum Foo {
  FOO_A = 1,
  FOO_B,
  FOO_C = 4
};

The syntax I went with is the following:

type foo :: {:a, 1} | :b | {:c, 4}

The only real alternative I could think of would be something like this:

type foo :: :a = 1 | :b | :c = 4

I don't like this, since it's nonsensical in normal Elixir.

I originally thought a list would be okay, something like this:

type foo :: [a: 1, b: 2, c: 3]

But this doesn't allow for partially explicit definitions, unless all the explicit definitions are at the end (to make it a valid keyword list), so the above definition isn't possible:

type foo :: [a: 1, :b, c: 3] # this doesn't work

I think the desugared pseudo-keyword list is a decent option. It seems less weird than writing :a = 1 to me, anyway. It could also be defined as a list (e.g., [:a, 1] | :b), but that seems confusing since [type] is meaningful elsewhere.

[mat-hek]

👋 thanks for the PR 😉 Personally I find quite unobvious that the alternative of tuples maps to an enum 🤔 What about

@type foo :: enum(a: 1, b: 2)
@type foo :: enum({:a, 1} | {:b, 2})
@type foo :: enum_value(:a, 1) | enum_value(:b, 2)

I like the last one the most, except that enum_value is extremely long. WDYT @FelonEkonom?

[harrisi]

I agree it's not the best syntax, so I'm happy to see more suggestions.

Something all three of your suggestions highlight is the use of the word "enum". I think it would be helpful to change the spec dsl to use enum instead of type in general.