Promote (comment) to special form

[iacore]

JANET_SLOT_SPLICED + empty array is used here.

The implementation is splice of empty array. It kind of works. When used at top-level, it returns @[]. (it's only visible in REPL though)

Is there a way to return JanetSlot that represents nothing?

Related: #1272

[CosmicToast]

To clarify the effects, since I quickly tested the patch, noting none of this is surprising to me:

1. In the top-level, this returns @[], but does not error. I would say this is fine, since it's in the top-level it's meaningless. (comment test) # => @[]
2. In a function-call, it's equivalent to nothing: (defn x [a b] (+ a b)) (x 1 (comment test) 2) # => 3
3. In a macro-call, it is unevaluated, and thus remains as-is: (defmacro x [& body] (pp body)) (x 1 (comment test) 3) # => (1 (comment hm) 3)

This is ultimately a breaking change (you can no longer use (comment ...) to mean nil), but is notably better than the ignore macro suggestion, as it does not error in the top-level. Perhaps it make make sense to keep the current comment while reusing this logic under a different name?

[bakpakin]

This isn't going to be merged for a number of reasons. This does not belong in the compiler and should be part of the parser.

[iacore]

Well, this is a proof of concept. I don't know how to add it to the parser (yet).