Recognise mapArray as a tracked scope

[jfrere]

Previously, mapArray was not seen as a tracked scope. Now, the first argument to mapArray is seen as a tracked scope, but the second argument is not.

See #52 and #55 for details.

Fixes #55

[jfrere]

I've just realised that I wrote "createMemo" instead of "mapArray" in the PR title... The code was correct, but clearly I was too tired when I was writing this up! 😅

[joshwilsonvu]

Haha, no worries, thank you so much for sending a PR! I'm especially grateful for the new tests. There's a little bit more I'd like to add since we're on the subject, namely:

• marking the second mapArray argument as a "sync callback", like the way Array#map callbacks don't open up a new "scope"
• handling indexArray as well

But I won't make you deal with that, just making a note to myself. This looks good to merge as-is!