add field_map function

[Jamiras]

Allows iterating over fields of a class instance, providing a way to convert a psuedo-enum into a lookup for use in rich presence:

class LookupItem
{
    id = 0
    label = ""
}

class DifficultyEnum
{
    function current() => byte(0x1234)
    function is(d) => this.current() == d.id

    none = LookupItem(0, "")
    easy = LookupItem(1, "Easy")
    medium = LookupItem(2, "Medium")
    hard = LookupItem(3, "Hard")
}
Difficulty = DifficultyEnum()

rich_presence_display("{0}", 
    rich_presence_lookup("Difficulty", Difficulty.current(), field_map(Difficulty, (k, v) => { v.id : v.label }))
)

achievement("Something Hard", "Do something on hard", 5,
    Difficulty.is(Difficulty.hard) && do_something()
)