Add binary-search exercise

[pfertyk]

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[pfertyk]

@BNAndras as discussed in #28, if the value is not found, we expect to receive a null. I've checked that the original Array.bsearch method in GDScript returns false in the same situation, so we're not far off :P

[BNAndras]

Do we want to return false then? It makes sense to match what’s already in the standard library. On the various Lisp language tracks, that’s what they do.

[pfertyk]

Thanks! I've updated null to false before merging :)