Improve depth-cutoff for printing HsSyn in error messages

	MERGE TO STABLE

The "user style" in Outputable allows us to elide large expressions
when printing HsSyn, printing "..." instead.  This is done by calling
Outputable.pprDeeper.

But there was no mechanism for trimming very long lists, which
occur when using do-notation or explicit lists.  This patch fixes
the problem, by adding Outputable.pprDeeperList.

I also made some of the pretty-printing in HsExpr rather more
vigorous about increasing the depth; in particular, pprParendExpr.
This should make debug prints shorter.