Add parens around implicit string concatenations where it increases readability (#3162)

Adds parentheses around implicit string concatenations when it's inside
a list, set, or tuple. Except when it's only element and there's no trailing
comma.

Looking at the order of the transformers here, we need to "wrap in
parens" before string_split runs. So my solution is to introduce a
"collaboration" between StringSplitter and StringParenWrapper where the
splitter "skips" the split until the wrapper adds the parens (and then
the line after the paren is split by StringSplitter) in another pass.

I have also considered an alternative approach, where I tried to add a
different "string paren wrapper" class, and it runs before string_split.
Then I found out it requires a different do_transform implementation
than StringParenWrapper.do_transform, since the later assumes it runs
after the delimiter_split transform. So I stopped researching that
route.

Originally function calls were also included in this change, but given
missing commas should usually result in a runtime error and the scary
amount of changes this cause on downstream code, they were removed in
later revisions.