add examples for keep

[rwtolbert]

add examples for keep

[sogaiu]

@rwtolbert @bakpakin

Although the last example sort of works:

# operates on values in table
(keep (fn [x] (when (> x 1) (* x x))) @{:foo 1 :bar 2 :baz 3}) # -> @[4 9]

I'm not sure it's such a good idea to advertise this.

Depending on the Janet version, the result may differ:

$ janet
Janet 1.37.1-60d9f977 linux/x64/gcc - '(doc)' for help
repl:1:> (map + {:a 1 :b 2})
@[2 1]

$ ./build/janet
Janet 1.28.0-dev-909c9060 linux/x64/gcc - '(doc)' for help
repl:1:> (map + {:a 1 :b 2})
@[1 2]

The docstring mentions "indexed collection ind" which arguably applies to "array and tuple" as indexed?'s docstring is:

Check if x is an array or tuple.

But should that be the case for a table or struct?

[rwtolbert]

Yeah, I just added it because it "worked" but I can see the advantage of making it fail in the core and then remove this example.