Add RFC for Choice.

[wanda-phi]

Rendered

[whitequark]

Guard syntax bikeshed:

1. Just stuff it in the list:

Choice(foo, {
  (1, guard): bar
})

2. Context manager:

with Choice(foo) as c:
    with c.Case(1, if=guard):
        c.eq(bar)

3. Explicit Guard primitive:

Choice(foo, {
  Guard(1, if=guard): bar
})

with m.Switch(foo):
    with m.Case(Guard(1, if=guard)):
        ...

(I don't love any of these. They're also all backward compatible.)

[wanda-phi]

Guard syntax bikeshed:

1. Just stuff it in the list:

Choice(foo, {
  (1, guard): bar
})

That won't work (Value is not hashable)