Relax dependencies for lens-examples; fix Brainfuck example.

The Brainfuck example fell out of date with Zipper becoming indexed.

Signed-off-by: Austin Seipp <mad.one@gmail.com>
Rename example executables.

From {brainfuck,pong} to lens-{brainfuck,pong}

We don't want to be rude and assume we're the *only* brainfuck
interpreter.

Signed-off-by: Austin Seipp <mad.one@gmail.com>