Fix unquote-splicing lists within quasiquote

[shawwn]

• The following expression now correctly returns '(x y z):

    (eval `(let (a 'y b 'z) `(x ,,@'(a b))))

(It was returning '(x y) since (unquote a b) was ignoring b.)

• MAP now supports splicing. E.g.:

    > (define-global spliced (x)
        (if (hd? x '%splice) x
            (obj? x) `(%splice ,x)
          x))

    > (define-global flatten (l)
        (while (first obj? l)
          (set l (map spliced l)))
        l)
    > (flatten '((1 2) 3 (4 5 (6))))
    (1 2 3 4 5 6)

    > (define-global intersperse (a bs)
        (tl (map (fn (x) `(%splice (,a ,x))) bs)))
    > (apply cat (intersperse ", " '(foo bar baz)))
    "foo, bar, baz"

[shawwn]

I'm not sure this PR is worthwhile, but the root problem was that I couldn't implement pg's continuation-passing macros:

Specifically, the ,,@parms was silently truncating parms to its first value. E.g. if parms was (x y z), then ,,@parms was giving (unquote x), which caused some very strange bugs.

This unquote problem is also present in arc (but interestingly, common lisp handles it) so perhaps this "bug" could be considered a feature for simplicity.

The %splice trick in map was an interesting way to solve this. Once it was added, the fix was two lines of code. The tradeoff is that map might become very slightly slower, which may or may not be an acceptable tradeoff.