[Feature request] Enums

[Macro Assembler AS](http://john.ccac.rwth-aachen.de:8000/as/as_EN.html#sect_3_1_14_) has enums, similar to the ones implemented by pret's [`const`](https://github.com/pret/pokecrystal/blob/master/macros/const.asm) macros. They define a series of numeric constants with a configurable starting value and increment value.

```
; rgbasm
foo equ 1
bar equ 3
baz equ 5
```

```
; AS
enumconf 2
enum foo = 1
enum bar
enum baz
```

```
; pret
const_def 1, 2
const foo
const bar
const baz
```