Lots of optics

[b-mehta]

Draft:

• It's unclear how efficient Batch is. It's also unclear if it (and its applicative instance) are defined in the correct way: it might allocate less if the order in :*: is flipped, and the <*> definition adjusted accordingly.
• Some of set, set', set'' might not be useful.
• The withXXX functions aren't consistent in their return type.
• There are some very bad names, eg MyFunctor and inn.

[b-mehta]

I've modified the Batch type as described above, and it is more efficient now.

[aspiwack]

I've pushed most of this PR to smaller PRs (#194 , #195, #196 ).

What's left is

• The set'' primitive, see #197 .
• The traversal story.

I'm leaving the PR open, still, as an inspiration as I'm trying to tackle the traversal story. See also #190 (and soon more).